Sunday, 17 March 2013

475kHz Transverter - an update

At last, today, I was able to put my 23cm and 13cm preamp building on one side and concentrate on my homebrew 475kHz transverter. In the few minutes I have been able to devote to it over the last few weeks I had become frustrated by low receive sensitivity and IF breakthrough from the 80m band. This was mean to be a therapeutic break from microwaves.

I had already decided to use a 3.2MHz conversion crystal, as G3XBM had done in his simple 475kHz transmit converter, for the conversion oscillator to the 3.675kHz IF. Indeed it uses the same circuit configuration as Roger. No Butler oscillators here!

However, whilst Roger's popular design passes the IF through to, e.g. an FT817 on receive, I wanted to use my K3 as the IF since it was already connected to the P3 spectrum display and via the in-built PC sound card to the K3 data ( line in and out) facility. A K3 doesn't normally cover down to 472kHz. This necessitated a complete transmit and receive converter effort as I couldn't find any suitable designs elsewhere.

I decided to use a Norton amplifier RF stage with 2N5109 followed by a five pole Chebi LP RF filter. Together these give a nice, flat topped bandpass characteristic with roll off below 300kHz due to the Amidon 68-2 transformer in the RF stage and above the LPF knee at 530kHz, thus allowing some coverage of the frequency range around 475kHz whilst providing excellent out of band rejection . No filtering is used in front of the rather 'strong' RF stage.

The mixer is a low level (+7) MCL SAM1. This may be changed to a +17mixer in due course.

Miniature relays are used to switch the signal path into and out of the mixer.

The low sensitivity turned out to be due to 'finger trouble'. This problem was also exacerbating the IF breakthrough. It was due to connector termination problems on the PCB groundplane. The ground wasn't properly connected........this happend as a result of a small design change the unwanted effect of which then went unnoticed at first.

With this problem corrected, but the transverter board still outside the screening case, there is still some evidence of IF breakthrough from 80m, as previously described ( elsewhere), but it is now MUCH reduced and the first genuine 630m signals have been heard. I was astounded at the strength of the BIA NDB this evening! A number of CW and digital signals have also been copied.

Most pleasingly, the bottom loaded antenna doesn't appear to be picking up much noise from the shack or house on 475kHz.

Now to do some further development of the transmit path.

73 de Sam

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