2304 MHz Transverter - A Little Bit About the TX Mixer

While I'm waiting for the parts to come to repair the ABPM, I thought I'd write a little something about the mixer that I'll be using with this rig.  The mixer I'm going to use is an old Vari-L DBM-184 flatpack double balanced mixer.  The specs on this device, although it is a rather old device shows some promise according to the datasheet.  It will accept up to a +20 dBm LO, the LO to RF isolation is about 20 dB, and is good to 2500 MHz on the RF port.  The conversion loss is about 8 dB, which is typical for a DBM.  Its 3 dB compression point is +6 dBm, which I would assume is the output power, which corresponds to an IF drive level of +14 dBm when  used with a +20 dBm LO.  I will not be driving this device that hard though, figuring on driving the device with about 0 dBm, and between +10 - +13 dBm on the LO port.

I picked up 2 of these mixers at the 2010 Packrat (Mt. Airy VHF Club) VHF Conference.

I am in the process of packaging the mixer in a brass box, just slightly larger than the mixer itself (3/4 inch square), with SMA connectors connected to all 3 ports.  This will make the unit a "module" which is how I like to build things, especially when working with the microwave bands.  This method of construction allows one to substitute different components, so one can experiment to find the best performing module for this particular combination of parts.

Basically, the planned signal path will be something like this:
From the LO tap in the CalAmp RX converter, LO will be amplified using a MSA-0386 in a homebrew amplifier run at saturation, feeding the DBM-184 mixer.  The IF will be an attenuated 144 MHz TX signal from the FT-817nd, feeding 0 dBm into the IF port of the mixer.  The RF port will feed a 3 resonator BPF designed by G0ORY, details of which appear here.  From the filter, the RF will be amplified by the already constructed ERA-2 mmic amplifier.  From that point, I'm unsure as to what the final power output will be, but I am hoping I will find an amplifier that will provide a couple of watts on the band.  What I use will depend on what the pocketbook will allow, as well as what power supply I have available.

Once again, stay tuned, as work progresses, I will post again.

2304 MHz Transverter - Transmitter Construction

After what seemed like forever, the ABPM (All Band Power Meter) arrived from Down East Microwave.  I had also ordered 2 ERA-2 amplifier boards at the same time.  I had the power meter built that evening, and went to test it, and the microwave detector does not work.  The parts in that section are extremely tiny, and I'm thinking I popped the detector chip.  I think it was probably my fault.  I also believe I'm losing my touch, as I'm having more difficulty working with surface mount parts due to their size.  I think I need to invest in one of those big magnifyers with the light in the head if I plan on continuing to work on this sort of thing.

Anyway, the power meter will read from about -20 dBm to +10 dBm in 2 bands - HF thru 500 MHz, and from 500 MHz to 10 GHz.  The low frequency sensor is working perfectly.  I got this shipment yesterday.

Today, I built one of the ERA2 dual mmic amplifiers.  I am going to use this as a transmit amplifier to get to the 10 milliwatt level.  10 milliwatts is a magic power level, as many higher power amplifiers are designed to input 10 milliwatts to get you from 1 watt to 100 watts, depending on what you are planning.

The ERA2 preamp boards are tiny as well, but I didnt have too much trouble building this one.  The 0805 chip caps are small, but fortunately, in these amplifiers, they're not too close together, and the circuit for a mmic amplifier is very simple.  These boards are nice high quality boards, made of Teflon, and have plated through via's.  These amplifiers provide about 30 dB of low level amplification, and will output up to 20 milliwatts at the 1 dB compression point.

This is a photo of the amplifier board.  As you can see, it is quite small.  Of course, I have no way to test it until I have the mixer finished.  I am currently working on that now, but I'm short one SMA connector to make that happen.

I still need to finish the mixer, and I also need to make a bandpass filter to place after the mixer that will pass 2304 MHz, but block the 2160 LO and 2016 MHz image.  I am going to use a 3-resonator type filter similar to the one I made for my 1296 rig.  Things start to get rather small on 2304 MHz, the resonators are less than an inch long on this band.  Its a good thing I have a micrometer.  I'll have the filter swept before I use it on the air.  If it's not selective enough, I'll use a cavity filter instead (or in addition to).  2304 is an awkward band in that it is more difficult to homebrew components such as the filters.  Cavities for 2304 are about 1 1/2 inches high, which makes them somewhat on the large size if you are trying to build small.  The resonator filters are less than an inch, and they require some level of precision for them to work well.

Once I get the mixer and filter built, I'll post again, and let you know how it all plays together.  Once I get to the 10 milliwatt level, I may attempt to make a contact on the rig if I can find someone with the capability to operate that band without having to drive too far.

Stay tuned, the next update will be coming soon!

New Project - 2304 MHz Transverter - Tuning the Front End

OK, I got the ambition to hook it all up today and start tuning the receiver.  I used a very crude setup.  The only signal source I have that will generate a signal near 2304 MHz is the second harmonic of the local oscillator in my 1296 transverter.  I used that as the signal source.

I had to set up the signal source about 20 feet away from the workbench to keep the RF from being picked up directly on the 13 cm circuit board.  I put a homebrew 1/4 wavelength antenna on the input side of a 0 - 50 dB attenuator, then fed the other side of the attenuator into the RF in on the converter.  I used my FT-817 as the IF.  Tuned to USB, I easily spotted the signal very close to 144 MHz on the IF.  The signal was about an S-2 with 0 dB of attenuation inline with the little 3 cm long antenna.  I began snowflaking the hairpins, adding small strips of copper to the hairpins where I saw an increase in signal and soldered them in place.  There were a couple places that gave huge increases in gain, in the middle of the filter.  After about 2 hours work, I was able to get the signal up to about an S-7 with 20 dB of attenuation in line with the little whip antenna I was using as a pickup.

The frequency stability appeared to be quite good, there was some drift when I first powered it all up, but that quickly settled down after it was on for about 5 minutes or so.  I did not have to touch the VFO dial in the IF rig after  that time.

The next step will be in building the transmit side of the transverter.  I am still waiting on my order to come in from Down East- one of W1GHZ's power meters and a couple of MMIC low level amplifier boards.  I need the power meter in order to tune the TX filter which I have yet to build.

Oh, I'm using this article as a guide to my conversion: http://www.qsl.net/g0ory/2.3g/31732/31732.html  No sense reinventing the wheel, being Adam already did the hard part.

Stay tuned, when I start on the TX portion, you can bet I'll talk about it here.

New Project - 2304 MHz Transverter

Its been so long since I've posted in here, I almost forgot I had this blog site.  I haven't been very active since my last post, but I feel its time to document a new project that I just started today.  This is a major project for me.  What I'm doing is building up a transverter for 2304 MHz, partially from scratch, partly from an old MMDS receive converter.  I'm building this unit around the California Amplifier model 31732 MMDS downconverter.  These units were originally used to receive over the air cable television.  I have successfully retuned one for Amsat Oscar 51 (AO51) ham satellite S band downlink on 2401.200 MHz.  The converter is extremely sensitive and they appear to be stable enough to use as a basis for this project.  This is the first time I've attempted to build my own microwave rig from either converting surplus or scratchbuilt.

The design goal for this transverter is to build a portable 13 cm station.  Power output of a couple of watts, and later perhaps as much as 75 watts using converted surplus.  This band is easy to do this with, being there is a nice variety of surplus hardware that can be used on these frequencies.  The project will use the MMDS converter for the entire receive chain, including the LO.  A probe (first modification) is installed just above the LO filter striplines, and exits the chassis to an SMA connector to be used to drive the TX mixer.

The TX mixer, bandpass filter and low level transmit amplifier will be homebrew, external to the MMDS whitebox. A surplus double balanced diode ring mixer will be used as the transmit mixer.  From the RF port, a homebrew 3 resonator filter will pass only the LO + IF output of the mixer, then will be amplified to approximately +10 dBm, which will drive a commercial PA, which, depending on what I use, will run anywhere from 2 to 75 watts, depending on the model I'm able to acquire.

The first modification I have done was to install the TX LO probe to tap off some 2160 MHz energy to drive the TX mixer.  This involved milling down a lip  on the case of the converter and installing an SMA connector with a 30 mm probe inside the whitebox.

Today, I received the necessary crystal to change the frequency of the LO synthesizer to output the 2160 MHz.  The crystal frequency is calculated by taking the operating frequency (2304) minus the IF frequency (144), then dividing the result (2160) by 256 (8.4375).  This is the frequency of the crystal I ordered.  I ordered this crystal from International Crystal, and it came in today.  I installed the crystal, and checked the frequency, and it netted on frequency with very little effort.  The output from the probe seems to be enough to drive the TX mixer.

The next step will be in tuning the receiver printed circuit bandpass filter, lowering its frequency from 2500 MHz down to 2300 MHz.  My next post will describe the procedure, and will keep you informed as to my progress.  So until then, I'll have to wait until I get some RG59 and some connectors so I can hook up the IF rig to the unit and start adjusting the hairpins on the pc board.

Cell Phone Interference on the Softrock Series SDR

Ever since I built my first Softrock, I was plagued by the usual buzz sound caused by a cell phone in close proximity to the radio.  It was so bad with the rig, that the phone could be anywhere within about 15 feet (or more) and the interference was still quite noticeable.  I had some success reducing it slightly by putting ferrites on the cables going to the radio, but could never reduce it to the point where it would not be annoying - until today.

The following modification should work on any of the Softrock sets, although I tested it on a TX/RX 6.3.

On an old scrap circuit board in my junk box, I found some molded RF chokes.  The particular value I found were 4.7 microhenries.  I installed one in series with the B+ line at the power jack in my shielded metal chassis thats holds the radio.  This reduced the interference substantially.  I then bonded the B-, shield of the serial connector and the grounds of the 2 audio connectors as well as the ground of the USB cable in my 6.3, and that reduced the interference to the point where I no longer hear the interference with the phone in my pocket, about 3 feet away from the unit.

I tried the inductor on the B- side of the power supply also, but when I did so, the interference came back to about the same level as it was before I began.

As an added bonus, by making this modification, I was able to also reduce the center "hump" substantially as well.  My analyzer screen in PowerSDR is now almost totally flat across the passband of the sound card.

If you have this type of interference on your V9 or 6.3, I strongly recommend doing this mod.  I assume the inductor value isn't critical, so long as the value is relatively small.  It seems that there is not enough capacitance between windings on the inductor I used to pass the 800 MHz garbage.

Progress Made on 1296 Rig

Well, we figured out the problem with the 1296 rig, but I'm unsure how to fix the problem.  Apparently there has been no oscillation problem all along, the local oscillator chain is producing harmonics that are causing some strange mixing products.  We had a strong output on around 1250 mhz that was always at the output anytime the transverter was keyed.  This signal goes away when the oscillator is detuned so that it stops oscillating, but is always there anytime the oscillator is going.  The signal levels are equal to the desired 1296 signal when driven by the IF rig.  So when the rig is running, I had an output on 1296 and a carrier at 1250 (or thereabouts).  We traced the problem to a dirty LO chain. 

We were able to clean it up substantially by inserting a 3 pole BPF between the 10 mW transverter output and the next amplifier stage.  The correct way to fix this in my opinion would be to incorporate another of these filters in the output of the LO chain.  We didnt get that far with it today though.

When we hooked up the 2W PA, there was no output from the amp, and we didnt have time to troubleshoot it.  It was working when we began testing, so I am guessing it is something simple, such as a short in the RF path somewhere.  Since the TX is now clean, I can now troubleshoot the amp and try to maximise the output power.

Another possible problem with the dirty LO is that of receive performance.  With a dirty LO, the receiver will be receiving on 2 different frequencies, cutting the gain down by 3dB (or more) on the desired 1296 MHz, and also increasing the noise by 3 dB (or more),  I never really noticed a receive problem with the exception of the RF stage oscillating, which was a confirmed problem.  So, with that discovery, work can begin cleaning up the LO, and eventually I should get a 1296 rig on the air , hopefully before summer.

1296 Transverter Progress

For quite awhile, I have been battling a low power problem with an older circa 1995 Down East Microwave 1296-144 transverter.  At first I thought the PA was blown, I removed it and installed an outboard PA with no success.  Eventually I replaced the entire transmit chain with still no improvement.

Most recently, I replaced the mixer IC.  The original mixer chip is no longer made, however a newer model which spec's somewhat better than the original was installed.

I never reinstalled the PA module.  I am going to do some testing very soon with the +10dBm output from the driver.  If everything checks out, I'll continue to use the outboard 2W module, then feed that into the 30W PA.  Keeping the rig this way, in my opinion would be more versatile as far as making improvements and upgrades on the circuit.  Because everything is interconnected using UT-141 copper hardline, things such as bandpass filters and amplifier replacements can be more easily tried.

I am hoping to put the rig on a spectrum analyser and power meter to determine if the transverter is working properly.  If it is, I plan on operating some 1296 up on the Lake, as well as possibly some mountaintop operations this summer.  Stay tuned to my blog, I'll report here on the performance once I am able to make the proper tests.

Softrock 6.3 RXTX with USB Control

I ordered and built the USB I2C controller board for my Softrock 6.3 RXTX.  Adding this option opens up a whole new world for your Softrock.  Not only does it allow you to tune the entire bands, but also allows you to have general coverage receive capability with the rig.

It also allows external programs to take control of the radio.  This is REALLY cool, as you can run Powersdr in the background and use software such as fldigi and WSPR to actually control the frequency.  Fldigi logging works great too, as it can read the frequency of the radio and it puts that into the log automatically.

Another added feature is you no longer need the serial cable to key the transmitter.  This is all done now via the USB port.  Of course, you still need to set up virtual com ports so the additional software can communicate PTT to Powersdr. 

I have been using PowerSDR v.1.19.3.15 with this new setup.  This version has been designed for use with CURRENT firmware in your softrock, and it works quite well.  I find the audio is less choppy with this version as well, and it's a lot prettier on the screen with its customizable skins.

Upgrading your Softrock with this little $11 board is more than worth adding the option. 

On a sidenote, the other night I thought I would try some digital communications running barefoot 1 watt from the radio, and worked a fellow in Arkansas using Olivia 500/16 on 40 meters.  I was running 1 watt, he was running 40 watts and was using a Flex-5000.  We maintained contact for 1 1/2 hours, talking about SDR in general.  Was a real fun QSO.  I'm looking forward to working my first Softrock to Softrock rig, as I think that would be quite interesting.

Softrock and the Digital Modes

Today I was experimenting with setting up PowerSDR-SR40 for use with fldigi and other digital software, and I got everything talking to each other.  When everything is working together, it makes operating digital so easy.  So far, I got PSDR working with fldigi and Ham Radio Deluxe.  I'm planning on getting it working with MMSSTV and EasyPal Digital SSTV program.  I dont think there will be any problems, as they should set up the same as the ones I already got working.


There are 2 programs you will need in order to make everything play nice.  First is "Virtual Audio Cable". it can be downloaded from here: http://software.muzychenko.net/eng/vac.html


When you get it, you will need to create 2 virtual sound devices.  There are good instructions at the above website on doing this.  One will be needed for receive, the other for transmit.


The other program you will need, and you will only need it for software that also controls the radio, such as fldigi and Ham Radio Deluxe, and that is "com0com", This creates virtual com ports to link the control aspects together.  It can be gotten here: http://com0com.sourceforge.net/


You will need to create 1 pair of com ports for most applications.  There are excellent pdf directions on how to do this at the above website.


Then go into Powersdr and set up the "CAT" section.  Although some software will work using the identifier set for "SDR-1000", I've had better luck setting up emulating a TS-2000.  Under CAT control, set the com port as the lower com port number that you created in the virtual com ports, I used COM5. Fldigi will use COM6 under its CAT settings.  I used RIGCAT in Windows to control the SDR, and it plays nice.  I have had some problems getting the external programs to control PTT, so I just use VOX for that.  I used to use VOX in my Yaesu too when I ran digi modes.


To get audio to the programs, you need to set up virtual audio cable.  I used Virtual Audio Cable #1 for audio out from PSDR and VAC #2 for the TX input audio.  These settings will be found in the VAC tab in the audio settings in PSDR.  The audio settings in fldigi are just the opposite - VAC2 is the output device, and VAC 1 is the input.


Once everything is talking, fldigi will read the frequency of the radio and you can then use the logger in fldigi, and it works like any other radio would with CAT control.


Ham Radio Deluxe sets up in a similar fashion.


To listen to your receive audio, should you choose to do so, VAC has a little utility called an Audio Repeater.  You can set that up to input the proper VAC device as the input, and your soundcard that you use for your speaker audio as the output of the repeater.


There you have it, a basic overview as to how to set up different software so they will talk to eachother.  Once they talk, operation is a breeze.  I worked a little bit of the RTTY contest, and worked about 25 contacts in less than an hour with only 15 watts of TX power.


Good luck, and I hope this helps you get things working with your Software Defined Radio.

SoftRock v6.3 RXTX+Xtall

About 2 weeks ago, I finished building a SoftRock v6.3 RXTX+Xtall SDR transceiver.  I ordered the transceiver and  3 power amp, low pass TX filters, so I can cover 80 meters thru 10 meters.


The 6.3 TXRX is a 1 watt PEP transceiver capable of running whatever frequency bands that you supply PA's for.  The standard kit allows for 16 presettable (with a 4 section DIP switch) frequencies.  Each frequency is tunable over  a span of 96 KHz with the software, with the preset dip switch setting being the center of that tuning range.  All modes are possible, including SSB, AM, NB FM, and all of the digital modes, including Slow Scan Television using outboard software.


Most of my operation so far has been on WSPR on the 40 meter band, however I have operated SSB on 20 and 40 meters, and have worked a couple of countries using an outboard HFPack amplifier at about 15 watts.  The performance is nothing short of remarkable, and have gotten exceptional audio reports while using a VERY cheap computer microphone.


Operating an SDR transceiver requires the use of 2 different sound cards.  I use one card that works as the digital signal processor for the radio and the other to drive the speaker on receive and mic for transmit.  For the radio card, you want the best sound card you can get, with the lowest noise and highest sample rate.  92 khz sample rate cards are available with noise floors in excess of -100 dB, and they are common and inexpensive.  I bought a Soundblaster Audigy card for this purpose, but with many soundblaster internal cards, there is some phase shifting in the card, and I couldnt get a good balance without setting the phase and amplitude all over the place.  I've used an older USB card such as the Soundblaster Live external card and that works pretty well.  The external cards apparently do not suffer from the phase shifting that the internal cards do.


With the bare transceiver with the fixed frequency settings, the only software that is useable with it is a special version of PowerSDR, called;PowerSDR-SR40 and is available at  http://powersdr-sr40.sourceforge.net/.  All other versions do not allow you to transmit if the fixed frequency is enabled.  There is, however an add on module that allows USB control of the Si570 oscillator that allows continuous tuning, which I ordered but have not received yet.


I will write more stories on this radio, and will detail a little more precisely on setting up this radio for use.  It is a fun little rig, and for roughly a $100 investment, you will have yourself a very capable little radio.