Sunday, October 31, 2010

2304 MHz Transverter - Significant Progress Made #1

Welcome back!  Today was a good day.  Got a LOT done on the transverter with the help of WA3TTS.  There was so much that we did, I can't write it all in one post, or in one night.  I'm going to split this up into several postings.  Question is, where to start.

First thing, and probably a very important thing is we checked the LO level and spectrum.  The level is much lower than I expected, I'm getting about -3dB at the SMA connector on the receiver brick.  The good news is it is VERY clean, all spurs are down by more than 60 dB, and the nearest spur is over 100 MHz away from the intended LO freq.  My plans have changed slightly because of this.  Since I have 2 of the Down East ERA2 2 stage amplifier boards.  I plan on taking the unbuilt board, and cut it in half, making 2 single mmic boards.  I will then use one of these boards to amplify the LO.  These boards should have about 15 dB of gain, so I should get about 12 dBm out of the amplifier, enough to properly drive the mixer.  I'm certain that if i fiddle with the LO probe, I could increase this level, but since the LO is so clean, I am almost tempted not to mess with it.  At least I know how much RF is there, it wont be difficult getting the level up to where it needs to be.

While I was there, I finished the mixer module.  We added a filter (more about this on the next post), and measured -20 dBm at the RF port of the mixer.  We only used the available -3dBm from the LO source.  A signal generator simulated the IF, tuned to 144 MHz.  The mixer output saturated at +6 dBm.  I believe that when I increase the LO level to +10 - +13 dBm, the output level will increase dramatically.

We did not test the 2 stage low power amplifier.  It is ironic to think however, that the -20 dBm coming out of the mixer matches perfectly the 2 stage gain of 30 dB, and in theory, I would end up with the +10 dBm that I'm shooting for at the output of this amplifier.  I believe it would be better, however to drive the LO to the +10  dBm level and run less IF energy into the mixer, thinking the linearity would probably be better coming out of the mixer.

As you can see from what is presented here, the rig is almost ready to hook to an antenna.  All that it needs is a good filter following the mixer, and perhaps a bit more power.  Tomorrow I'll write about a filter that we retuned and some of the really cool features of this filter.  We'll also talk a little bit about power amplification too.  I'll leave the cliffhanger here... Lets just say, I got a solid state power amplifier that will provide meaningful power for portable work.  It will need some work to get it to run on 2304, so will be experimenting with that soon also.

Stay tuned, keep the dish pointed this way, and check this frequency soon for the next update.  73 for now.

Thursday, October 28, 2010

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.

Saturday, October 23, 2010

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!

Saturday, October 16, 2010

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:  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.

Thursday, October 14, 2010

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.
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