ozQRP MST400, Руководство по монтажу

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MST Construction Manual – Issue 1 Page 32
8.5 C
ARRIER FREQUENCY ADJUSTMENT
The suggested carrier frequency is 10.000MHz which places it just above the passband and on
the upper skirt of the crystal filter frequency response. The frequency response shown in Figure
12 was obtained from a completed prototype MST board. Note the passband is approximately
2.5KHz.
Due to tolerances in components the response of the crystal filter in your board may be shifted
slightly to that shown here. This can result in one of the following.
1. If the response is shifted higher in frequency to the right, the carrier frequency is within
the pass-band and the upper sideband is not suppressed sufficiently.
2. If the response is shifted lower in frequency to the left, the carrier frequency is too far
away from the pass-band and results in ‘thin’ sounding voice due to the lack of low
frequencies.
If you feel this is the case then the carrier frequency may need adjusting. There are a number of
ways to go about this but the easiest is to listen to a good strength station of known quality
while gently adjusting trimmer capacitor TC1. Adjust until the received audio sounds natural
and the balance of high and low audio frequencies is correct.
Alternatively monitor yourself on a nearby receiver while speaking and adjust the carrier
frequency until you sound normal. You can also switch between sidebands on the receiver to
hear the reduction in the upper sideband. Note that you will need to adjust the VFO frequency as
you make the adjustment as the received signal will be moved off frequency by an amount equal
to the change in the carrier frequency.
Figure 12 Crystal filter response
If you move the carrier frequency off 10.000MHz and are using the companion DDS VFO then
you will need to measure and re-program the IF frequency to ensure the display reads
accurately. See the DDS VFO construction manual for details.
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9985000 9990000 9995000 10000000 10005000 10010000 10015000
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Frequency Hz
Crystal Filter response