KVN SSB Filters used in superhet receiver

KVN Filter PCB. SSB Receiver.
See an earlier post for schematic and details of the Telrad board. Also called the Israel board due to the location of the supplier, I believe.

HF Receiver for SSB.
After 6 months of construction and experimentation I am not really happy with the result of my efforts to create a ‘decent’ receiver from this board. The 1496 mixers are great as product detectors or modulators, but as front end mixers, they leave a lot to be desired. I believe that the whole board was designed to work at higher signal levels than the sub uV signals that we hams are used to listening for.
Furthermore, I really wasn’t too keen on the dual power supply requirement as I only have 12v DC available and the few inverters that I either built or bought were noisy.

Download the PDF Telrad filter SSB receiver

 

I have removed the filters from one of the boards and used them in the design of a superhet receiver that works fine. Some of the parts I had at hand may not be available to everyone. However, just replace that building block with your favourite circuit. The pic is not quite the same as the schematic. The schematic is up to date. There was excess gain in the receiver, so I have removed the post AM filter amp, and also the audio preamp. I built this receiver into the same case that housed the TELRAD board, along with a second TELRAD board that was the ‘potential transmitter’. I also used the TFT touchscreen display and Si5131 VFO synth, controlled by an Arduino MEGA. Having used a Hybrid Cascode kit for the I/F amplifier, and no audio filtering, the whole thing is a bit more compact than the original design. There are a number of spurious signals emmanating from this setup, and as the screen is being constantly scanned by the Arduino MEGA for touch activity, there is also some noise being generated there. I really like the touch tuning on the screen and fine tuning with the encoder, but basic RF performance outweighs the ‘bells & whistles’ so its back to a ‘quieter’ LCD 2 line display. For a receiver, rather than a transceiver, this means a smaller case is possible.

The size difference is startling. I used a NOS case from a PRC351 ex military VHF transceiver, and it certainly has a military look about it.  I have reduced the band coverage from 9 bands to 4 bands in order to minimise the BPF. I also used the smaller Arduino, the UNO with an AD9851  based DDS. The UNO is overclocked to 28MHz, providing a bit more processing speed than the MEGA. See an earlier entry in my blog for information on overclocking the Arduino.

Update Sept 18-2017   I miscounted the I/O requirements for the control of the receiver by not including the band select lines. I have now decided to use an older Teensy 3.1. This is small and quite fast. It’s 3.3v so I can see further interface problems looming. Oh the joys of homebrewing !!

Main Up/Down tuning will be via the keypad. Fine tuning with the encoder. That leaves just the volume control needed on the front panel. Bandchange is actioned via the keypad with the Teensy programmed for correct mode per frequency. There are a number of spare buttons for extra functions. The rear of the front panel is a shielded box containing the control system and VFO. The back panel holds Anderson power poles and an audio jack. There is a BNC socket for the antenna, mounted in an existing hole in the left side of the case. I will post more details on this mini receiver, along with the schematic, once I have finished building and testing.
It won’t be too much different from the above drawing.

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