925 MHz Transverter

I have finally completed (re-)building my 925MHz transverter which has been a work-in-progress project for the last 2 years.    In the early ninties I built my first transverter with limited test equipment;  alignment and testing was done using an attenuator, VCO Module and a homebrew stripline wavemeter.    Thankfully it did work and I successfully made several contacts during VHF contests.   One niggling issue was it was extremely sensitive  to power supply voltage:  It worked well at 13.8V but not at 12V  as there was not enough drive for the class C PA module.

Since acquiring proper test equipment I decided to give it a tune-up to get it running reliably and discovered a number of problems.    Lets just say it was easier to scrap the original and start over again using a board from Paul Wade W1GHZ.

In order to complete the system a number of extra circuits were needed.  After some research I decided it was easier to build the supporting circuits myself as they were useful for some of the other bands I was also in the progress of building.

The final design consisted of the following building blocks:

W1GHZ Transverter

The design is based around the W1GHZ “No Tune” Transverter for 33cm.  As the primary allocation in the New Zealand bandplan was 20MHz higher the filters required realignment by trimming several mm’s off the hairpin filter striplines.

Si4133 Local Oscillator

I was unable to find a off the shelf crystal that would produce a clean 780MHz local oscillator using Paul’s 720MHz LO board, so I built a versatile Local Oscillator based on the Si4133 chip.   This LO is capable of generating any frequency between (approx) 750MHz and 1600MHz at +10 dBm – more than enough for directly driving the mixers in a typical transverter.

Sequencer

In order to interface to the IF radio the TX path requires approx 40dB of attenuation, and the RX path requires somewhere from 0dB to +10dB.

The photo shows the IF control attached to the sequencer and 8Vvoltage regulator

LNA

Initial tests of the transverter showed that the total conversion gain was close to zero dB, which can impact the overall system performance.   Adding a GaAsFET preamp to the front end allowed the noise figure to be set by the front end of the transverter and no longer compromised by the losses in the system.

Power Amp

This PA is based on the SHW5177 module available from rfextra on eBay.  After some experimenting and a sacrifice in the name of science trying to reverse engineer the correct bias settings, this is able to produce > 8W of power in class AB linear operation with around 5mW of drive.

Final Checkout

In order to test the complete system a trip was made to Hawkins Hill to test against the local 33cm beacon on 925.275MHz.    While I was able to hear “Jim” there was obvious problems with the front end.     The GaAsFET preamp was found to be unstable, and after adding extra decoupling and making a few refinements adding broadband termination  the low frequency TV and FM signals no longer causes oscillation the transvereter.

There is plenty of room to install the completed transveter in a Jaycar HB5050 die-cast aluminum box.

Findlay is showing in interest in Amateur Radio by helping test the transverter from Hawkins Hill:

Future Work

The filter used in the PA and LNA board is cheap and offers similar performance to the hairpin filter used on the W1GHZ transverter in a reduced size.     A future project is to combine my individual modules to build a single board 925MHz transverter…..

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