Icom IC-7300 AM Frequency Response

Discussion in 'General Technical Questions and Answers' started by KC2TAU, Apr 17, 2017.

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  1. KC2TAU

    KC2TAU Ham Member QRZ Page

    In operating my IC-7300 one thing that I noticed and has been noted by others as well is a rather noticeable low end roll off below 200hz on both receive and transmit. This results in rather flat sounding receive and transmit. A good way to experience this is set the tone controls at flat for both AM and SSB. Next, tune in a strong local AM station and note the bass response when listening in AM then switch to SSB and note the difference. Also due to this behaviour the bass tone control does not have anywhere near as much effect on both AM receive and transmit compared to SSB.

    The IC-7300 has a provision for providing a 12khz IF output from the USB cable. The manual explains this was intended for DRM reception. However, it can also be demodulated by using an SDR program and this provides a 12khz wide output of the receive section. When I did this one thing that I noticed is that the 200hz roll off did not exist when using this 12khz IF output.

    I then tried feeding audio back into the USB port such as one would do when operating digital modes. To my surprise I found that the 200hz roll off was not present on AM transmit as well.

    I spent some time looking at the service manual to see if I could gain any clues and the only thing that I was able to find was that the accessory port and USB cable are fed by a different pin on the D/A as compared to the microphone and speaker/headphone out. However, this doesn't tell me much as the bass roll off does not exist in SSB mode on either receive or transmit.

    Another thing I noticed is that Icom uses the same DSP libraries across several radios. I'm quite confused as to why Icom would implement a roll off that is only present on AM on both receive and transmit and yet the IF out is not subject to this. If anything, you hear about restricted frequency response more often on SSB than AM. I'm also curious as to where this IF out is derived from in the circuit. What is peculiar, though, is how if you change the mode or filter width on the radio when using the 12khz IF out you can hear the change on the demodulated audio but changing the receive EQ on the radio has no effect. I have been using SDRuno for demodulating the IF out and listening to the resulting audio.

    While all of this may sound like a minor detail the difference in receive quality when deriving the audio from the IF out versus the speaker/headphone out is quite dramatic. It turns the receive quality from something that can be quite tiring to a much more enjoyable and listenable experience.

    Any ideas or information regarding this would be greatly appreciated.
     
    Last edited: Apr 17, 2017
  2. KM3F

    KM3F Ham Member QRZ Page

    What you describe is very much the same as in the now old Kenwood TS 2000 design.
    It uses 12khz DSP the same way.
    The 2000 will reproduce receive audio way down as far as 20 hz and out to 5khz..
    The KW Tx audio through put is good for 20 hz to 3100 hz where is brick walls.
    I run outboard audio processing over this range and its outstanding for Tx audio.
    What your doing to gain fidelity loses selectivity that is done after your sample point.
    You can have both the way you do it.
    Its a matter of what you want from the radio at different times and modes.
    To illustrate the value of DSP in the KW, a frequency of 7.255 and an interfering signal at 7.25350 only 1500 hz down can all but be eliminated with the DSP set at 1800 hz providing the signal strength is not much above an S8.
    This sacrifices fidelity of the desired signal for interference reduction but that's the way it goes in Ham radio.
    Filtering in the earlier IF stages is very important to overall selectivity.
    You just added a way to add to the flexibility of the overall operation.
    I did replace my second IF filter with one that has more poles and sharper skirts.
    I might add that this also reduced the overall audio frequency response a small amount in favor of improved selectivity.
    You have something good going by the sounds of it.
    Good luck.
     
  3. K8JD

    K8JD Ham Member QRZ Page

    My Take on High Fidelity Amateur Radio Fone
    Communication grade (Or telephone grade) audio will eliminate what part of the audio spectrum that is not useful in transmitting intelegability?
    The very high , above 3 KHz and low below 300 Hz. part of the audio spectrum !! .
    It is a waste of power, to have a wide spectrum of audio, since the object of an amateur radio transceiver is to deliver intelegability under less than ideal conditions/
    Broadcast transmitters and receivers are designed for good fidelity, to convey music in an enjoyable form, Broadcasters employ high power to have a well received signal in their assigned coverage area. .and transmit a high fidelity spectrum from 20-20000 Hz !

    TWO COMPLETELY different objectives !
     
    Last edited: Apr 18, 2017
  4. KC2TAU

    KC2TAU Ham Member QRZ Page

    From doing a bit more work I am unsure as if my findings regarding the TX audio being sent through the USB port are correct. After reviewing the equalization curves that I have used to get more natural sounding audio I may have very well just been correcting for the roll off that exists. The IF out isn't being demodulated and is probably being derived before it arrives at the DSP.

    This is one of those interesting dilemmas about SDR technology. If you purchase something that uses it from a company that actively involves its users in the development process then it can be of great benefit as desired features can often be added with a few weeks of coding time. However, if the coding inside of the radio is much more closed off then this can be a become a bit of a road block. In decades past when radios were more component based if one did not a certain feature or behaviour trait of a radio then it could more than likely be modified in circuit to achieve the desired effect.
     
  5. KB1YFD

    KB1YFD Ham Member QRZ Page

    I thought the 7300 was limited to 100 Hz to 2900 Hz. I think this TX audio is bandpassed in software via TX synthesis. Does the USB audio in get handled differently than the mic input? I'd like to know.
     

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