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SSB, IF, BFO questions

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Full Member level 3
May 31, 2015
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I am a bit confused about SSB demodulation. Let's say we have IF 9MHz and have a filter for SSB with BW 3kHz. And also the frequency band that we are working on is 14MHz-14.350MHz.

1) If somebody says that he is on 14 200kHz and he is working with USB modulation that means that the signal uses frequencies from 14200-14203kHz?

2) Our IF filter passes frequencies from 8998.5 kHz to 9001.5 kHz. That means that it would only pass frequencies from 14200-14201.5 kHz if we would tune to frequency of 14200kHz?

3) That means that we either should have a filter that is centered on frequency 9001.5kHz or we would have to tune to 14201.5kHz?

4) Than we should have two filters, one for USB, one for LSB?

5) If I am using BFO to demodulate that signal what frequency should I use for USB modulation? And what for LSB modulation?

6) My understanding is that we should use BFO with the same frequency as IF frequency for USB, and use IF+3kHz for LSB?

I learned a little about SSB from doppler motion sensors patents and papers regarding RFID (active scatter, etc.) It may be a little different, but may help. Assume we have doppler motion sensor with constant frequency F. If wave reflects from approaching object, it frequency will get shift F+Fd (so it will be upper side band). If wave reflects from the object that moves away, it lowers frequency to F-Fd (lower side band). IF filter connects to each output of quadrature mixer, so it will pass any signal USB or LSB. For example, same object with speed V but moving in different directions give the same IF frequency. But output of quadrature mixer will have phase difference between IQ channels, so we can determine if it was USB or LSB.

That all rather depends on how you set things up...

Generally the LO is tuned such that the filter passes the correct sideband (The exception being in radios only supporting one mode where the filters are sometimes offset to the appropriate side, see some old military sets).

The BFO is then on the side of the filter appropriate to the sideband being used, hence why you used to see two crystals separated by a few KHz.

So for a filter centred on 9MHz, working USB on 14.2MHz nominal frequency, you would set the LO such that the audio was centred on 9MHz, which means a 1.5KHz offset, you would then inject the BFO 1.5KHz below the filter centre frequency.

Note: This assumes low side LO, in the rather more common high side LO the fact we are using the difference means that the sidebands swap over at the output of the first mixer.

Fortunately these days, shifting the LO relative to the displayed frequency is trivial.

73 M0HCN.

That's exactly what was confusing. Now you confirmed that we are really tuning to frequency+1.5kHz offset. In commercial receivers, is it done internally so we just pick 14200kHz but the radio really listens on 14201.5kHz for USB and 14198.5kHz for LSB, or do we need to manually set that offset?

For this filter **broken link removed** we need to have LO to 4990kHz to receive audio centered on 14001.5kHz because 14001.5-4990=9011.5 kHz, which is filter's center frequency, if I understood correctly.

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Also, after the mixer, which goes first - IF amplifier or IF filter and do I need to have impedance matching circuit between those two?

On modern rigs it is done internally (Trivial for the micro to offset a number before programming the synth), older stuff usually did not have the precision for it to matter (If you were +-10KHz, that was as good as the dial got).

Where to put the IF filter is something of a tradeoff as placing it immediately after the mixer means the IF amp is only exposed to the energy in the passband so overload is less of an issue, but while not an issue on HF, it does hurt the system noise figure on VHF and above.

Ideally you want to place a wide filter after the mixer (wide so it has good overload performance) followed by a little fixed gain to make up the losses followed by the main selectivity filters, followed by the main IF amp.

Note also that the mixer really should be wideband terminated on all ports, diplexers are your friends.

Yes, you should be paying attention to impedance matching (Quarts filters are normally hundreds of ohms, not 50, a simple L match will usually suffice....

73 Dan.
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If I use this filter, it has input and output impedance 200 ohm.
If I place it after the mixer than my mixer should have output impedance 200ohm? Then after the IF filter I have this dual gate MOSFET with AGC as a IF amplifier. And I am confused with if I need impedance matching circuit between IF filter and IF amplifier? Dual gate MOSFETs have high input impedance and IF filter has 200 ohms output impedance.

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Sorry, in my last post I gave a link to a filter that is for SSB. It should have been wide band filter. SSB goes after the IF amplifier. In fact, ignore my last post, I figured out those questions.

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I have other question instead.
he writes about transforming impedance in drain LC circuit
The drain inductor was replaced with a 10:2 ratio transformer to give the drain a load of 1.25 kΩ to work into when a 50 Ω load is placed on the amplifier output

Shouldn't he calculate that LC circuit impedance in resonance and then calculate turn ratio. For example it that LC circuit has 2k ohm impedance in resonance then he should have 40:1 turns ratio to give 50 ohm output impedance?

What is the difference in using 5:1, 10:2, 100:20 turn ratio?

Maybe this is a stupid question, but here it goes. On crystal filters, can I swap the input and output? Does that change filter characteristics?

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