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LF: Re: Vectorscope display

To: [email protected]
Subject: LF: Re: Vectorscope display
From: "James Moritz" <[email protected]>
Date: Tue, 19 Feb 2002 15:07:25 +0000
In-reply-to: <002001c1b8b2$602dfa80$1700a8c0@home>
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Dear LF Group,

I had the usual crop of bouncing e-mails, but I hope I have now sent the vectorscope circuit to all who have requested it. Thanks to G3YXM for putting it on his web pages - so hopefully it will also be available there shortly. Correction - I don't seem to be able to send it to [email protected], Johan, so perhaps you can get it from Dave's site later, or is there something else I can do?

Re: Andy's comment:
...how about
the next version, a direct conversion receiver generating quadrature signals
at 137kHz then having masses of low frequency gain before the X/Y plot ?

A low noise, high gain version of the circuit would certainly be possible, but there are a number of problems to be overcome here:

The signal spectrum at the mixer output extends to DC, so any DC offsets will be amplified as well as the wanted signals. This would saturate the output unless offsets could be kept down to a few uV. Even then, the remaining offset would effectively be an unwanted 0Hz "carrier" in the demodulated output spectrum. A related problem is that the noise figure of amplifers, etc. increases drastically at very low frequencies due to flicker noise. Both these problems could be avoided by having AC coupling, but this would put a hole in the received signal spectrum around the LO frequency.

Also, if we low pass filter the I and Q channels in order to define the bandwidth, any mismatches between the filters in the two channels would result in errors in the phase quadrature between the 2 channels, which would be a problem if good image rejection between the sidebands were required in subsequent demodulation of the signal.

I think it would probably be possible to achieve a useable result for a dedicated 136kHz RX - a fairly simple bandpass filter before the mixers could restrict the bandwidth to a few kHz, and some pre-mixer gain could be applied, reducing the post mixer gain required and reducing the effects of offsets.

Cheers, Jim Moritz
73 de M0BMU




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