[minima] Re: kerchucking noise in PTT
- From: David Fannin <dfannin@xxxxxxxxxxxxx>
- To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
- Date: Fri, 2 Jan 2015 18:59:20 -0800
Thanks,
I put the TX/RX power inputs for the BiDi IF amp, and the output of the IF
map on an oscilloscope, and toggled the TX/RX modes.
The popping/rushing noise is coming from the IF amp - I can also see that
the TX power decay takes about ~100 ms, while the RX power comes up within
a couple of ms. There is definite overlap for 80 ms or so while both TX
and RX circuits are powered up - The popping noise matches up with this.
I've attached a video of it in action.
Dave KK6DF
On Fri, Jan 2, 2015 at 6:41 PM, allison <ajp166@xxxxxxxxxxx> wrote:
> On 01/02/2015 04:22 PM, David Fannin wrote:
>
> Makes sense. Seems like good use for a 2N7000.
>
> Yes.
>
> Do I need any resistors between the Drain or Source and Ground, or can I
> just wire to provide a short to ground when switched on? Do I need R2 in
> the attached?
>
> R2 no. Put the drain lead on the other side of R1. that will limit
> peak current
> and still pull it down hard. I usually put a resistor in series with the
> 12V to the gate
> anything in the 10-100K range is ok.
>
> See if that helps.
>
> Allison
>
> Dave
>
> On Fri, Jan 2, 2015 at 12:41 PM, allison <ajp166@xxxxxxxxxxx> wrote:
>
>> On 01/02/2015 03:23 PM, David Fannin wrote:
>>
>> On Fri, Jan 2, 2015 at 11:51 AM, allison <ajp166@xxxxxxxxxxx> wrote:
>>
>>> On 01/02/2015 02:38 PM, David Fannin wrote:
>>>
>>> Ok, an update , but no solution yet.
>>>
>>> Here's what I've done so far:
>>>
>>> 1) changed the mic preamp C3 cap from 10uf to 1uf. (from my schematic,
>>> I'm using a NE5534 op amp mic preamp) - no change
>>>
>>> Does not match any schematic I have. You need to put up a AS BUILT
>>> schematic.
>>>
>>
>> Schematic for my mic preamp was posted earlier in this thread.
>>
>>>
>>> What I'd asked was to disable the output of the mic preamp and see if
>>> the TX/RX pop remained.
>>> That was what I'd meant by pulling C48 (V2 schematic).
>>>
>>
>> I disconnected both mic pre-amp and cw oscillator , so I cut the leads
>> of C48 and C47. That was #2.
>>
>>>
>>>
>>> 2) disconnected the mic pre-amp and cw oscillator from the audio
>>> pre-amp input - no change.
>>>
>>> 3) replaced the relay - no change
>>>
>>> I cannot imagine how the relay could be at fault.
>>>
>>
>> Someone suggested this, and I had used a socket for the relay and had
>> an extra one, so it was about 15 seconds of work to rule this out.
>>
>>>
>>> However when I do a DC switched TR circuit I usually return the
>>> alternate lead to ground so that
>>> when I go TX all the RX+ points are forced to ground and same from going
>>> to RX wher all the
>>> TX+ points are grounded. It can be done with realy or my preference a
>>> few transistors.
>>>
>>
>> Some progress based on this idea. On the BiDi amplifier, I disconnected
>> the 12v power input to the TX side of the BiDi amplifer (R23) , but left
>> the 12v input to the RX side (R65) . Both sides are switched by the
>> relay. The popping sound went away ( the sound is only going from TX to
>> RX, not RX to TX.). So with the current V2 circuit, when +12v/Xmit/R23
>> is deactivated, that point is isolated, not grounded.
>>
>>
>>
>> Yes, the logic is if caps are allowed to discharge into the circuit
>> they tend to take a while.
>> So if you take R23 to ground on RX it may dump the remaining voltage in
>> the circuit.
>> A simple transistor switch using the RX+ Voltage to switch on a
>> transistor on from R23
>> to ground might be enough. Also the .1 caps at the input and output of
>> the TX IFamp
>> are likely larger than needed try .01.
>>
>>
>> Allison/KB1GMX
>>
>>
>
>
>
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