PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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DSP-IF transceivers


The transceiver designs of the last decennium tend to make use of the ever increasing power of microprocessors.
Some use a DSP for several audio processing tasks, including the last IF stage.
Mostly, these transceivers use a verry low (around 30kHz) last IF, due to the limited processing-speed available.
Without a preselection filter (a roofing filter) these designs may proof to be less resistant to high signal environments such as multiple transceiver contesting setups.
But for the average ham they offer very nice, practical features, like continuously adjustable filter bandwidth.
A very steep conventional, X-tal or mechanic filter, always has ripples in its passband, and together with the ringing effect of such a steep filter the audio quality on transmitting as well as on receiving is doomed to be less than ideal.   
The absence of such a very steep conventional filter does improve the audio a lot, which can easily be observed listening to a well adjusted IC556 -like radio, or to a Software Defined Radio.
I did some measurements on my IC746Pro, which uses the same DSP as the IC756 family.

Testing your receiver's audio response:

There's a simple way to test the frequency response of your entire receiver chain.
Just put up a small local RF signal from any source and tune your radio to it in SSB mode.
Rotating the dial of your radio, you will hear the sinusoidal tone of that signal.
Turn the dial towards a lower pitch until it becomes just too low to hear. Try to get a "zero beat".
Read the zero beat frequency from your display. Now turn the dial slowly towards a higher pitch until the volume drops at the high end (often around 2,5kHz) Note this frequency as well.
Ok, during the sweep, listen carefully for increasing or decreasing volume on your speaker. 
Any big peak or notch is bad news, and should be investigated.
It will generally be caused by a poor speaker or poor listening position. Again good headphones will take care of this.
Small ripples within + and - 3dB are quite normal, and a slightly raised curve towards the high end is quite normal as well.
 The frequency response of your SSB filter is showed by your S-meter; small fluctuations again are normal, but in this measurement they will not result in ripples in your audio, since your automatic gain control will smoothen this automatically.
Fluctuations of more than 6 dB on your S-meter will however be audible in normal operation and are therefore not ok!

Some of the newer generation transceivers use DSP-based SSB-filters which can be user-modified. 
If it offers the choice between hard knee and soft knee filters, be sure to choose soft knee; a steep, hard knee filter will show much more ringing effect which is really unpleasant and may lead to listening fatigue much faster.)


The Icom 746PRO audio response measured:

Using wideband moise, I measured the audio frequency response with my tone control settings ( a half point low roll-off and high flat). Note that this curve is taken from the line output of the receiver, this means that speaker and room acoustics are kept out of the results:


 
The curve shows that:
1, The low end rolls off slightly below 100Hz.
2, The mid part shows a minor bump at 300Hz, the rest of the passband is pretty flat. 
3, At 3,3 kHz an extremely steep roll off is marking the top end of the audio spectrum. (SSB filter setting:Wide)

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Ok, let's measure the transmitting frequency response as well. 
Note that for the following measurement the line-input was used, 
which can NOT be adjusted by the IC746pro's transmit tone control:



 
This graph is taken with SSB transmit filter WIDE. 

 Again a very useful and almost ripple-free frequency response, due to the DSP filter, which lacks the ripples in the passband known from steep X-tal filters. This ripple-free response is the main reason why this generation of transmitters has fairly good audio, without any need for external audio filtering.

Note the roll-off below 140Hz.
For my cardioid condensor microphone, as well as my modified handmike, I additionally attenuated the low-end and boosted the high end.
This was done purely by "human ears judgement", on a separate receiver, and by precise and honest audio reports by hams.

My IC746pro transmit settings are:

TX bass: -3
TX treble: +3
Compression: off , 1 for poor conditions
Mike gain 60% (=1 o’clock)
Transmit Bandwidth: Wide for easy, high signal level QSO's
Mid for most QSO's in poor band conditions, and for DX, Pile-ups etc.
(Mid reduces low end mainly, and just a pinch of high end is reduced.)

Note that the IC746pro recalls the bandwith setting for both compression off and on situations.
Therefore, I put the settings so that with compressor off, I have maximum bandwidth, and with compressor on, 
bandwidth switches to the Mid setting automatically.
So when the QSO gets tough, I have to push the Comp button only to get compression plus reduced low end.
This works very convenient.

 


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