Testing a phasing SSB receiver: Part 1 – with a mobile phone

Testing a phasing SSB receiver: Part 1 – with a mobile phone

My latest project is this simple receiver for 80 meters its direct conversion unlike others it is actually a phasing receiver so it can cancel the opposite sideband providing single signal reception it's based on a design by SP 5a HT although his rigorous for a transceiver I finally made this one a receiver in this video I'll be using some simple test equipment to see how good the image rejection really is, first of all, a quick look at the receiver under test it's very simple just three transistors and one IC yet it's a phasing SSB receiver for 80 meters the local oscillator is in the left half of the case it uses to be c5 for a transistor it's free-running one transistor is the oscillator and the other is a buffer that output signal feeds into the balanced mixer which I'll talk to about in more detail in a minute.

Tthe incoming signal comes from the antenna socket via an RF gain control and that also acts as the volume control you don't need two of them in a simple receiver and then into an RF preamp from the output of the RF preamp the signal goes into an RF phase shift network just a capacitor and a trimmer resistor that splits the signal into two and feeds it into the diode balance mixer mixing it with a local oscillator signal from the vfo the male audio signal goes to an audio phase shift network comprising of a few capacitors and resistors that's combined and is then fed to an LM 36 audio amplifier so it's a very simple receiver to transistors in the vfo one transistor in the RF preamp and an lm386 IC in the audio amplifier yet it's a single signal receiver with opposite sideband rejection and is strong enough to drive a speaker on most signals our first piece of test equipment is an RF signal generator it's actually a DDS vfo a kit by in freeze ed I you can see here the main vfo board and the display board but there's extra room in the box for a few hour of things the kit was a joy to build and worked first time so I'd highly recommend it the other item of test equipment is actually a mobile phone app this one is called frequency and is available for Android phones it's free and you can download it and it effectively gives you an audio spectrum analyzer now turning 80 meters we have wiles of the right here is a 7600 we're now going to run a series of tests first of all internal noise the receiver is off the gain is set to full and I'll turn the receiver on that's the receiver long that's the receiver off so the receiver has very little internal noise next thing we'll do we'll connect the antenna just watch what happens on the phone now the noise has gone up to minus 60 on the display so there's clearly adequate sensitivity to my noise level which admittedly here is quite high it's 200 Hertz that's minus 32 that's gone up to 300 Hertz and it's about minus 22.

Sso there's a big change there which is which is good which is about how you want it it's been more at 400 Hertz it's pretty much level much level where – 20 dB but from Circular here top it drops dramatically which again is is about what you want hi there it's not quite smooth looks like we've got a bit of a double hump there because it's quite quiet 4 kilohertz and very high at 5 kilohertz so it's a bit of a hump around two and a half kilohertz but it really drops off about 60 kilohertz you can still hear it around 7:00 to 10:00 so it's quite clearly need some improved audio filtering above about three or four kilohertz because we really shouldn't be hearing audio and that sort of level you can hear very clearly the difference in the signal the strongest signal you heard was in the correct lower side then whereas this receiver is set up to reject the upper sideband and it was significantly weaker the receiver is set up to 3.5 stories zero megahertz sir as the n3 said I DVS hence nothing is audible from a receiver as we are exactly zero beat the signal generator now is 3 for 99.5 so it's a 500 Hertz time and it's rating around 22 DB minus 22 DB on the phone that's our reference we're now we've now gone up one kilohertz 35.5 and that's about – 32 DB so that's around 10 DB rejection there's a trim pot in the RF phase shift network and if we adjust it see what effects it has on the opposite on the image and it dramatically drops the volume of the image we're on three four 99.5 350.50 round 20 DB of image suppression be at 500 Hertz three point four nine nine 5501 and it's reading the tucked under – 40 so again that's a 20 DB suppression almost just adjust the raising pot now look at that that is spectacular about 30 DB of image rejection at one kilohertz which is pretty good for something so simple but of course one of the costs of a simple phase shift network is that the image rejection varies across the frequency range so we'll try another frequency like 2 kilohertz three four nine eight long city P 502 it's minus 50.

I haven't adjusted anything and we're 20 DB down or just is more twittering and it's down there nearly at minus 60 back to the lower sideband it's minus 30 so again at 30 DB rejection, I did have to Toodle the phasing control so it's not quite uniform across the whole audio frequency range okay so it's not quite a single signal reception but even a very simple phasing receiver can rejects and wondered signals by maybe 20 or 30 decibels it's not great for the competitive DXi but for the casual listener it is a big improvement over a simple direct conversion receiver that gets no image rejection at all I'm suggest looking on the web for we many transceiver projects by sp5 an HT it's an incredibly simple phasing 80 meter SSB transceiver and this receiver is really a simplified version of that there are some performance compromises but it's still a pleasure to use and in the trade-off, between simplicity and performance I think it scores pretty well.

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As found on YouTube