Archive for September, 2018

Tx inhibit: how to simplify the TX/RX sequencing

Posted in Electronics, VHF on 21 September 2018 by iw0ffk

When I started to prepare my home 10 GHz station, the designated transceiver for this band was the Yaesu FT-817, because I knew that many microwavers use it as a IF for their transverters but mostly because I already had it and pratically unused.
The little Yaesu has the input signal “TX Inhibit” on one of his ACC pins, so I used it in my Eyal-Gal TX-RX sequencer of a previous post.
The TX INH operation is simple, if you put a voltage on this pin (12V), the transceiver will never transmits radiofrequency even if keyed.
So it was enough to use a couple of wires that bring the PTT and the TXINH signals from the transverter on the rooftop down to the FT-817 in the shack.
When the PTT gets closed, the transverter starts the RX->TX sequence, the TXINH is at +12V and no RF power comes out from the RTX. At the end of the sequence, the TXINH is switched to GND and the transmission can start safely. Same story for a mast mounted preamplifier instead of a transverter. Also a linear amplifier can be keyed in that way, in order to assure a cold switch (doesn’t work on CW full-break-in of course).

Why this approach is MUCH better than the common sequencers?
The answers are:

  1.  because there is no need to wire the microphone, Morse key or PC PTT to the sequencer box anymore: the sequence starts when the transceiver closes the PTT, so everything has to be connected on the transceiver in the usual way.
  2. because one can safely use the features of the transceiver like voice keyer, cw keyer and memories, rtty macros, auto tuning etc. without damaging the device on the rooftop.
  3. because it’s more “fail-safe”. For example with the usual sequencers one can accidentally press the key “SEND” on the transceiver’s panel and the preamplifier’s device is fried in a millisecond.

Really I don’t know why Kenwood and Icom doesn’t have on their rigs this great feature that helps who use preamplifiers and/or transverters and/or linear amplifiers. Anyway it’s easy to add an external circuit to obtain the same result. This schematic is for my 10GHz transverter that works with an IF on 70cm, it’s just an example.
tx_inhIn few words a negative voltage is sent to the transceiver’s ALC input to inhibit the RF output until the device on the rooftop is fully switched to TX.
I tested this circuit on a Kenwood TS2000 that needs an ALC voltage of about -7V to completely cut off the RF output. The Icom rigs needs about -3V, in this case the 68k resistor and the trimmer have to be sized to meet the ALC limits. The DC-DC converter can be replaced by a 9V battery. The calibration is very easy: connect the transceiver to a dummy load, transmit in FM and turn the trimmer until the RF output power is completely choked.

The video below shows the TX inhibit intervention at every RX to TX, the ALC indicator goes to the full scale (no RF output) then it’s released and the RF can come out. In CW one has to send a “dot” before transmitting, as a conventional sequencer.

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New SDR receiver in the shack

Posted in Electronics, VHF on 3 September 2018 by iw0ffk

airspy

Recently I bought an SDR receiver in a dongle form to replace the old RTL-SDR in use on the IF of the transceivers, in order to have a panoramic view of the band.

The dongle is Airspy mini that has a good balance of quality and price, 12 bit ADC, samplerate up to 6 MSPS and an acceptable dynamic range.

My purpose is to have a detailed view of the noise floor on the 10GHz band to detect any change when a cloud usable for rain scatter is pointed with the dish.

The previous RTL-SDR was connected to the intermediate frequency of the transceiver, just after the first mixer. This time, thanks to the suggestion of Pino IK0SMG, I moved the RF tap to the input, after the front-end and the filter.

Actually I use a Kenwood TS-2000 with the 3cm/23cm transverters that are placed on the rooftop. The transceiver is tuned to 432 MHz and 144 MHz respectively on the converted frequencies of the two microwave bands.

Fortunately the TS-2000 has a TMP connector already mounted on the TX-RX 2 board, where the VHF and UHF paths are joined before to enter in the first mixer. This connector (CN15) is used in the factory for the band pass filter adjustment and it’s perfect to tap off the signal for the SDR without desensitize the main receiver.

cn15

I’ve done some test and the spectrum can be zoomed-in down to a dB per division.
I taken a short video of the 2m IW0FFK/B beacon. The beacon is controlled via Telegram messages, first it was set to “CONTINUOUS CARRIER MODE”, then different RF attenuators have been inserted. The SDR measured amplitude match the values of the attenuators! FB

If you want to send commands to the beacon, join the beacon’s Telegram group by clicking this link.