Spectrum DSP M2 User Manual page 23

What to do if you notice that the ALC or AUDIO meters jump when you
push PTT:
In a quiet room with an antenna or dummy load connected to the Spectrum DSP, set the
METER mode to ALC and key the microphone/transmitter without talking and note if ALC
meter jumps at the instant that you key the transmitter and goes down again. Next, switch the
METER mode to AUD and key the microphone/transmitter again, watching the AUDio meter.
If you notice that either meter jumps upwards when you key the transmitter and drops down
again your keying the transmitter may be causing either an electronic "click" or mechanical
"clunk", "de-sensing" the transmitter's ALC. This can be caused by the the powering-up of the
electret element in the microphone when the radio is keyed and/or by the (noisy!) mechanical
action of the switch – but the result can be the same in either case: A temporary "desense"
when you start talking and/or an annoying sound heard by the station receiving you!
To minimize this adjust menu item "TX Mute Delay" which will keep the microphone audio
muted for a short period after keying up. The parameters are adjustable from 0 (off) to 25,
which keeps the audio muted for a full 250 milliseconds (one-quarter of a second) after the
microphone is keyed.
It is recommended that one finds the minimum value to reliably suppress the appearance of
the microphone key-up noise and then increase it by 50%.
Comments when using AM:
AM transmission operates the same way as SSB, but frequency translation mode
•
must be activated. Remember also that the unmodulated carrier in AM will be ¼ that
of the PEP power in SSB!
Important information regarding the "Frequency Translate" mode:
Menu item "RX/TX Freq Xlate" selects the enabling/disabling of baseband frequency
translation in the receiver/transmitter. When the translation is active, instead of the receiver
operating at and around "DC", the signals are mathematically shifted from 6 kHz (above or
below – user-selectable). Whether or not frequency translate mode is enabled is displayed on
the start-up splash screen.
Performing this frequency shift can help forgive a lot of the "sins" that occur with "DC"
conversions - the most obvious of which are that ANY noises in the power supply as well as
the 1/F noises of op amps, mixers, A/D converters and the like tend to show right up in the
received audio. With the signals at microvolt levels, it is a real fight to minimize these signals!
These signals/problems can show up as:
• Hum
• Howling
• Audio feedback, particularly at higher volumes
Spectrum DSP M2 User Manual V1.1
P 23