dc to 4 MHz am/ssb
receiver
An old Thurlby frequency counter module was to hand and although it had a number of IF offset pre-selectable possibilities, non were very useful for a simple general purpose receiver. Since this counter over-flowed at 4 MHz and standard ceramic resonators were available at this frequency, it was decided to use this as the IF frequency for the receiver. For a wide frequency coverage receiver using a simple low pass filter before the first mixer, this would normally imply a lpf roll-off start frequency of about 3.5 MHz, but to enable the 80m amateur band to be covered, it was decided to compromise and start the roll-off at 4 MHz and add notch filters at the IF frequency prior to the first mixer to avoid bleed-through of signals at that frequency.
For various reasons, it was also decided to use a direct conversion IF frequency detector (if that isn't a contradiction). Once advantage of this is that it allowed the receive bandwidth to be set by a simple switched capacitor digital audio filter after demodulation. All that was required to do this was to make the clock frequency variable. For this to work on AM as well as SSB, it meant having to use an appropriate AM detector topology, but this was just another opportunity to try something different.
As a result of these considerations, the following block diagram was arrived at:
The greyed boxes are not detailed here, but details can be found elsewhere:
1. DDS: Just Google 'VK5TM'.
2. Diode ring mixer: Bog standard +7 dBm type
3. 4 MHz quadrature detector: Google 'Finningley SDR' (the 15 Mhz xtal is replace with 16 MHz one)
4. Audio power amplifier: An ebay Chinese manufactured TDA7292 module
4 MHz lpf with IF trap and RF amplifier
Again, using the calculator app at the rfcafe website https://rf-tools.com/lc-filter/ for the low pass section:
 |
 The smd inductors result in fairly obvious in-band loss as the frequency approaches the 4 MHz knee, but the image response at 8 MHz and above remains pleasingly high.  In order to use up reclaimed inductors and still get close to the nominal calculated value, the board allowed for three to be placed in series, though this was only required for one string, hence the two 0 ohm links. |
It would be nice to forego an RF amplifier, but it can always be taken out later and it does bring the sensitivity up to a more reasonable 2uV or so. The mmic used was an MSA0385 and has a gain of just over 10dB. |
 |
Mixer and IF filter
An SBL-1 diode ring mixer was chosen because of its high IF/LO port isolation spec (>65 dB at the working frequency). A simple three pole 4 MHz ceramic filter follows the mixer to provide a reasonable degree of out of pass-band rejection, easing the signal handling requirements of the following IF stages.
 |
 |
SSB detector
A few Finningley SDR bare boards were left over from that project, and using one at the 4 MHz IF frequency felt interesting. The input LPF was not fitted and nor were the DC blocking capacitors on the I and Q outputs, allowing the DC present to set the biasing for the op-amp stages on either side of the polyphase audio phasing network shown below. The values were made such that full opposing sideband rejection up to 6 khz could be obtained, particularly as a number of amateurs are beginning to use wide-band SSB.
 |
Having previously
half-hardheartedly played with a polyphase audio
phase shifter, this project seemed an ideal
opportunity to have another go. All the
resistors and capacitors were measured and
selected from a larger lot (250, in the case of
the 10n capacitors). This allowed matched
components to be obtained at about 0.25%
accuracy or better. |
Adjustable corner frequency audio Low Pass Filter
The unfiltered audio on ssb sounds very nice when the surrounding channels are quiet, but it was always going to require some sharp selectivity, so a good compromise is to have two options switchable. For the low pass filter, a MAX4703 switched capacitor filter IC was chosen. This gives a shape factor of 1.2:1 with a stop-band minimum of 60dB - quite acceptable figures. Because the corner frequency is set by a clock frequency 100 times higher in frequency, it is easy to make that edge variable by changing the clock frequency, and to do this, all that is required is to have a variable capacitor shunting the clock pin to ground. |
 |
AGC
On amateur bands, reception is often better without AGC in that you get a more realistic feel of things like QSB, but on medium waves say, where there is a much wider signal strength range, you can obviously get some very painful audio shocks as you tune onto a stronger signal, so AGC becomes essential in those cases. Just as the low pass filter has made to be switchable in and out of circuit, so then has the AGC circuitry. An SL6270 has been used, but with the second stage reduced down so that the AGC effect can be had milder, giving a less pronounced 'wall of sound' effect. To get an optimum balance, a pot has been used to allow some experimentation. So far, no 'pumping' effect has been noticed with AGC switched in, and the application circuit time constant values have been kept, since these seem to work well here. |
 |
AM detector
This was a chance to try a quadrature detector using an FM IF strip, but with separate paths applied to the ICs mixer. The limiting amplifier was fed from a narrow band crystal filter to reduce the modulation content of the signal. Instead of feeding the second port of the chips mixer from a quadrature phase shifted version of the limiter output, it was fed directly from the mixer/IF filter pcb. A quick play with a couple of ebay 4 MHz quartz resonator resulted in the response opposite - the trimmer allows some control of the in-band bandwidth. This would be a good start for isolating the carrier from an AM signal prior to limiting. |
 |
*I'm having a bit of trouble with the
limiter/quadrature detector - a good idea gone too far
perhaps*
