DC Receiver Tuning

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In most CW transceivers we typically transmit around 800Hz LOWER in frequency than we receive. Looking at it another way we receive a frequency 800Hz higher than we transmit.

The reason for the above dual explanation is because of various radio specifications.  For example the specifications for the HW-8 tells us the transmit frequency is 750Hz lower.  In the case of an IC-725 the displayed frequency is the transmit frequency and receive is 800Hz higher as painstakingly found by calculations in the service manual.  In either case the rule above applied.

For a Direct Conversion (DC) receiver, like the HW-8, if a signal is being transmitted at 7.100.000Hz we can tune our radio VFO to either 7.100.800 or 7.099.200 to hear the 800Hz tone (7.100.800 – 7.100.000 = 800Hz audio OR  7.099.200 – 7.100.000 = 800Hz audio).  A superhet or some of the newer phasing type receivers one of the images are filtered out.

Note again we usually transmit LOWER in frequency than we receive.  So now we will do a little math scenario…….

We hear a CW station calling (he’s actually transmitting on 7.100.000Hz) and tune our radio from the high frequency side down until we get our favorite 800Hz note.  Our DC VFO will be at 7.100.800Hz (7.100.000 transmitting frequency – 7.100.800 VFO = 800Hz audio).  When it comes time to transmit we will be transmitting back on 7.100.000 (7.100.800 – 800Hz shift lower = 7.100.000) and the calling station hears us.

If we instead tuned our DC radio from the low frequency side and get that same 800Hz tone our VFO would be at 7.099.200Hz (7.099.200 – 7.100.000 = -800Hz).  Now that our VFO is at 7.099.200Hz and we go to transmit our transmit frequency will be 800Hz LOWER and will become 7.099.200 – 800 = 7.098.400Hz or 1,600Hz lower than the other guy is transmitting on and an even 2,400Hz lower than that magical 800Hz note he is listening for.  The receiving station may hear you as a very high pitch if he has a wide DC receiver with no filters.  If he has audio filters or uses a modern SSB type radio which filters the lower sideband out, he will probably not hear you at all.

You can change these values a little for your own radio and hearing preferences. For example the HW-8 claims a 750Hz shift lower in transmit.  Both my HW-8’s are 1,000Hz lower (apparently a little tweaking is needed!).  You may also like to hear 600Hz as your audio frequency but still transmit 1,000Hz lower.  You can do the math for these tweaks but will see they still come out close to where the other guy will hear an audible note or can re-tune with his RIT control on the “high side” but is still way off on the “low side”.  The moral of the story…. For HW-8 users (and as explained in the HW-8 manual) tune from High end of the band (250 on the dial) to the Low end (0 on the dial).  If you pass zero beat, back up!


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