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G4AXX

Receiver Protection



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Specifications
Frequency Range DC-50 MHz
Max Receiver Level +8dBm
Size 90mm(L) 35mm(W) 30mm(D)
DC Power 24Vdc @ 80mA Tx, 0mA Rx
Weight 180g


These two protection circuits are for use in multi-transmitter contest stations. They protect receiver inputs from being overloaded by close-proximity high-power transmitters.

They were used in the GU8D station in IOTA 2005 to allow in-band cross-mode multiplier working, fitted to both FT1000MP & FT1000 MkV transceivers.


Click on a thumbnail below to see a larger picture

The schematic or, click here for it in PDF format (12kb). A PTT-controlled relay switches the antenna between the transmitter output and the receive-only RX ANT IN input of an FT1000MP transceiver.

Two units, for the Run & Mult stations. The relays are Jennings RJ1A vacuum relays. The separated receiver input is protected by back-to-back PIN diodes.

The cable connections. There is a diode-or circuit on the PTT circuit so it can be fed to another piece of equipment if required.



Relay make & break times


The pull-in time, between applying 24V to the coil and the Tx connecting to the linear. 2.23ms and 2.33ms were measured on two relays.

The break time, between removing 24V and the Tx disconnecting from the linear. 1.59ms and 2.82ms were measured on two relays.


Intermodulation due to the PIN diodes

The PIN diodes protect the RX ANT IN input from high voltages from on-site transmitters, but do they compromise the receiver performance in contest conditions?

Two signal generators were combined to measure the intermodulation products produced by the PIN diodes. Carriers of 14.110 & 14.190 MHz, both at 0dBm, were applied. This picture shows the spectrum without the PIN diodes.

The spectrum with back-to-back PIN diodes shunting the input to the spectrum analyser. The 3rd-order intermodulation products are -57.8dB down on each carrier.


This gives an IP3 (Third Order Intercept Point) of +28.9dBm (0 + 57.8/2 dBm). Now a signal level of 50uV (-73dBm into 50R) should, on HF, read S9. How high will the intermodulation products be with signal levels of S9+40dB (-33dBm)?

The PIN diodes will create intermodulation products at (-33 - 2 x (28.9 + 33) = -156.8 dBm. That's a long way down in the noise floor on HF.


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