[K2] Receiver overload protection

[vk2rq]

When building the K2, one installs a couple of back-to-back diodes across the input of the IF amplifier to clamp the input voltage. However, if there is a large signal on the antenna during receive, what protection is there for the RF preamp transistor and the first mixer? Does the micro controller monitor the V_RFDET line on the antenna during receive, and switch out the preamp/switch in the attenuator when a large signal is detected?

73,
Matt VK2RQ
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[Don Wilhelm-4]

Matt,

There is no provision for that as far as I know, however the 2N5109 used
in the K2 preamp stage is quite a robust transistor.  For instance the
absolute maximum base to emitter voltage is rated at 3 volts and the K2
preamp design raises the emitter 1.3 volts above the board ground.  It
would take a very healthy signal on the K2 input to produce greater than
3 volts peak at the base to emitter junction.
Even the K2 Receive Signal Tracing instructs one to inject a 0.14 volt
RMS signal for tracing purposes, and that is a very strong signal (I
calculate 39 mW).

The 1st mixer (a TOP1 diode ring mixer) is capable of handling very
large signals as well.

I would not drive a 5 watt signal directly into the K2 receiver and
expect no damage, but under normal operation there should be no problem.
If you are expecting an extremely large signal on the feedline while the
K2 is connected, I would suggest that some external protective device be
added to your station.

73,
Don W3FPR

On 5/2/2014 2:32 PM, Matt VK2RQ wrote:
> When building the K2, one installs a couple of back-to-back diodes across the input of the IF amplifier to clamp the input voltage. However, if there is a large signal on the antenna during receive, what protection is there for the RF preamp transistor and the first mixer? Does the micro controller monitor the V_RFDET line on the antenna during receive, and switch out the preamp/switch in the attenuator when a large signal is detected?
>
>

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[vk2rq]

Thanks Don. I was chatting to a fellow KX3 owner about how the KX3 protects itself in the presence of strong RF, and he remarked that my K2 would probably do the same. When I looked into it, I was reaching similar conclusions as you, although I didn't find any information about maximum signal ratings for the mixer.

BTW, 39mW corresponds to 16dBm, which is indeed huge. I calculated 0.14Vrms to be more like -4dBm (ie. an S9+69dB signal), but maybe there's a stray decimal point, or I've missed an assumption somewhere.

Anyway, best thing is to take some measurements, and see if an external protection device is warranted. Thanks to Steve for the diode suggestion when using separate receive antenna.

73,
Matt VK2RQ

> On 3 May 2014, at 5:06 am, Don Wilhelm <[hidden email]> wrote:
>
> Matt,
>
> There is no provision for that as far as I know, however the 2N5109 used in the K2 preamp stage is quite a robust transistor.  For instance the absolute maximum base to emitter voltage is rated at 3 volts and the K2 preamp design raises the emitter 1.3 volts above the board ground.  It would take a very healthy signal on the K2 input to produce greater than 3 volts peak at the base to emitter junction.
> Even the K2 Receive Signal Tracing instructs one to inject a 0.14 volt RMS signal for tracing purposes, and that is a very strong signal (I calculate 39 mW).
>
> The 1st mixer (a TOP1 diode ring mixer) is capable of handling very large signals as well.
>
> I would not drive a 5 watt signal directly into the K2 receiver and expect no damage, but under normal operation there should be no problem.
> If you are expecting an extremely large signal on the feedline while the K2 is connected, I would suggest that some external protective device be added to your station.
>
> 73,
> Don W3FPR
>
>> On 5/2/2014 2:32 PM, Matt VK2RQ wrote:
>> When building the K2, one installs a couple of back-to-back diodes across the input of the IF amplifier to clamp the input voltage. However, if there is a large signal on the antenna during receive, what protection is there for the RF preamp transistor and the first mixer? Does the micro controller monitor the V_RFDET line on the antenna during receive, and switch out the preamp/switch in the attenuator when a large signal is detected?
>

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[k6dgw]

Well ... I can tell you with absolute certainty, zero doubt, that a 500
W 80m CW transmission consisting on one dit can, and did, silence the
front end of the K3 across the room on 75m for the remainder of the
contest.  The antennas were probably in each other's near field.

Corollary Observation:  Bandpass filters [both rigs had them] are about
as effective when both rigs are on the same band as bluffing in solitaire.

My advice, if you're in a situation where your K<anything> "could" get a
high dose of RF is to use protection.  In this thread some time ago,
several of the usual suspects on the list who tend to be right most of
the time seemed to converge on 0 dBm as the absolute limit.  YMMV.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org

On 5/2/2014 2:59 PM, Matt VK2RQ wrote:
> Thanks Don. I was chatting to a fellow KX3 owner about how the KX3
> protects itself in the presence of strong RF, and he remarked that my
> K2 would probably do the same. When I looked into it, I was reaching
> similar conclusions as you, although I didn't find any information
> about maximum signal ratings for the mixer.

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[Don Wilhelm-4]

Matt,

Yes, I moved the decimal in the wrong direction!  Your calculation is
correct.
If you have the K2 connected to an antenna near other antennas that is
being used for transmit, then I would strongly recommend that you either
disconnect the antenna from the K2 (or KX3 or any other receiver) during
the time you are transmitting.  That could be accomplished automatically
with relays, but if you are not doing SO2R, manual antenna switches will
do that task just as well.

73,
Don W3FPR

On 5/2/2014 5:59 PM, Matt VK2RQ wrote:
> Thanks Don. I was chatting to a fellow KX3 owner about how the KX3 protects itself in the presence of strong RF, and he remarked that my K2 would probably do the same. When I looked into it, I was reaching similar conclusions as you, although I didn't find any information about maximum signal ratings for the mixer.
>
> BTW, 39mW corresponds to 16dBm, which is indeed huge. I calculated 0.14Vrms to be more like -4dBm (ie. an S9+69dB signal), but maybe there's a stray decimal point, or I've missed an assumption somewhere.
>
> Anyway, best thing is to take some measurements, and see if an external protection device is warranted. Thanks to Steve for the diode suggestion when using separate receive antenna.
>
>

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