Sunday, 31 January 2016

Tuning the receiver front-end of a Kenwood TKR-750 repeater

Tuning the receiver front-end of a Kenwood TKR-750 repeater

Introduction

I bought a second-hand Kenwood TKR-750-1 VHF repeater. However it was not clear what band the receiver was tuned to. The actual receiver and transmit frequencies can be programmed with a computer. However the the receiver band width is tuned with a spectrum analyser and tracking generator.

This post describes how to re-tune the receiver front end.

The band pass filters for a 2m repeater RX are quite critical as they are the only selectivity. With a 600 kHz spacing, the cavity filters are usually just isolating the TX and RX when sharing the same antenna, but providing no bandpass to strong nearby signals.

Starting point and instrumentation

I have an old HP8591A spectrum analyser and tracking generator, circa the 1990s, but still a capable instrument. The basic procedure is to connect the tracking generator output to the RX antenna input and the spectrum analyser input to a test point after the RX bandpass filters and amplifiers, just before the first mixer.

The pass-band of the receiver will then be shown. Before adjustment, the centre of the pass-band was about 160 MHz and the bandwidth about 10 MHz at 6 dB. This pass-band would make the receiver useless for the 2m amateur band, in Australia, 144 to 148 MHz. I was apprehensive if I could move it that far.


The receiver front end is relatively simple. From the antenna through two filters L2 and L3, a single RF amplifier bipolar transistor, then three more filters, L5, L6 and L7. The test point is CN1, at the bottom left.





The HP8591A is connected to the antenna and test point. However, before connecting the instrument, the output of the tracking generator must be reduced to about -30 dB. It took some searching through HP manuals to see how to do that, but it is easy when you know how.

Just in case I triggered the repeater, I connected a 50 Ohm dummy load to the TX antenna output.

The test setup shows the devices connected together. The five filters can be seen clearly.


Amateur radios use an odd connector, TMP from Taiko Denki, for interconnecting boards and for test points. Fortunately I had chased some up for when I did the IF tap on my IC-7410 TRX. They are $1-50 from the USA, so I bought ten. http://www.therfc.com/taiko.htm

The other must have is a RF coax connector adapter kit. The gold connectors going from BNC to N. http://rfshop.com.au/adapters-coaxial/adapter-kit.html and http://www.therfc.com/adaptor.htm They are a must-have for working with modern radios, from HF to microwave. The two connector ends screw together with a threaded barrel.



The adjustment was suck it and see, not having done it before. Using an insulated tuning stick, I started with the first filter, screwing it in as I wanted to lower the frequency. This adjustment showed up nicely on the spectrum analyser. I then went to the next in the chain and so on. I needed to change the centre frequency on the analyser so I could see where I needed to go. I went though the chain of filters trying to get the right shape, right frequency and minimal losses.

The end result: success

Eventually I had a nice pass-band centred on 147 MHz with the same 10 MHz bandwidth, pretty much the same shape as the original. I was relieved I was able to manage to move it sufficiently, otherwise the repeater was of no use.


RX selectivity and cavity filters

The main purpose of a duplexer is to separate the RX and TX to allow them to share the same antenna. As a reader of my blog will see my interest in building cavity filters. I found it quite fascinating that a repeater could transmit 50 W into the same antenna where it was simultaneously receiving micro Watts of RX signal.

On 2m, the spacing is only 600 kHz (recently increased to 1.6 MHz in Australia, but that is another problematic story). Bandpass cavity filters do not have enough selectivity for this narrow spacing. Special pass reject cavity filters must be used. The TX bank of three cavities pass the TX signal but have a deep notch at the RX frequency. Similarly, the RX bank pass the RX frequency but have a deep notch at the TX frequency.



However, while the duplexer allows the repeater to work, the duplexer is not a bandpass filter for the RX. All the selectivity is left to the little filters in the RX itself, making them critical if there are strong nearby signals.

In Australia, there is a pager band starting at 148 MHz, right next to the 2m repeaters.

On UHF, 70 cm, the spacing is higher and allow the use of simpler bandpass cavity filters. These high Q filters do the two tasks of providing selectivity as well as RX-TX isolation.

Conclusions

It is realtively easy to retune the RX frontend of a repeater using a spectrum analyser and attenuated tracking generator. Tuning the repeater RX on bands with close RX-TX spacing is quite critical as it is usually the only bandpass selectivity.

PS UHF TKR-850-1

Before; about 460 MHz centre, 8 MHz wide


After: OK but not quite as good, centre 434 MHz and 10 MHz wide; a 26 MHz move!






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