Contests
Equipment
Logsearch
QSL info
Links
Receiver Protection
|
Back to Equipment
|
 |
Specifications
|
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.