[Elecraft] Re P3 safe input level

[Victor Rosenthal 4X6GP]

Thank you, Mike. I will check the output of the 51J-4 on my scope. The 
IF output is from the last stage and there is an output amplifier that 
might have some gain, so I better measure it and add an attenuator if 
needed.

73,
Victor, 4X6GP
Rehovot, Israel
formerly K2VCO
CWops #5

On 19/07/2025 20:09, Michael Carter via Elecraft wrote:
> Hi Victor,
>
> There was some old discussion on the Elecraft forum about output levels from the KXV3 IF output port.  The P3 designer, Lyle  KK7P, indicated that the IF output level is about the same as the signal input level at the K3 antenna port.  I've not encountered S9+60dB signals very often, and that may be beyond what results in saturation of the P3 input circuit. You're probably safe up to 200 uV into the P3 (S9 is -73 dbm or 50 uV into 50 ohms, so S9 + 12 dB equivalent signal level).
>
> The P3 RF input circuit does not have any level-protective elements - an optional preamp is switched into that circuit, but otherwise the RF goes straight into the A/D converter via a transformer (single-ended to differential drive converter).  The A/D converter, an LTC2205-14, specifies maximum analog input voltage of [-0.3 V --  Vdd + 0.3V].  The P3 uses 3.3 V for Vdd for that A/D converter.  The A/D converter does have internal clamp diodes that limit voltages to prevent damage, but I would still put an inline attenuator from your 51J-4 output to the P3 as insurance.
>
> The P3 frequency coverage is specified from 455 kHz up to 21.7 Mhz, so your studies of the 51J-4 IF filters should be OK at 500 kHz.
>
> 73,
> Mike, K8CN
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