PreAmps and Crystal Filters

[psapengineer]

We're designing a new Rx combining system for VHF. The site is noisy, so in addition to the necessary bandpass cavities, we also need crystal filters on the carrier frequency.

So, here is the question: In which order do you prefer to stack your components?


Antenna, Arrestor, Junction, Critical Length Cable, Bandpass Cavities, Pre-Amp, Crystal Filter, and Rx

or

Antenna, Arrestor, Junction, Critical Length Cable, Bandpass Cavities, Crystal Filter, Pre-Amp, and Rx


I've seen it and done it both ways but I'm looking for more input.........

In advance, Thanks, Bob

[Wowbagger]

As a general rule, you want you amplification as early as possible, so that you have the best noise figure. The later in the chain you go with the amp, the more you are amplifying the noise floor and the less you are amplifying the signal.

HOWEVER: the problem with moving your pre-amp earlier is that if you get too much out-of-band signal and you saturate the amp, you will be amplifying the distortion not the signal.

You will have to measure the TOTAL SIGNAL at the output of the band pass cavities, and see if that signal is within the spec for the pre-amp. If it is, then you can use the pre-amp to make up the loss of the crystal filter.

If the pre-amp cannot take the signal at the band-pass filters, and you would have to put the amp right before the RX, then I would question the need for the pre-amp at all: true, you might be getting "more signal" to the receiver, but you are ALSO getting more noise, and I'd question if the signal to noise ratio was really being improved or not. Unless the receiver is really bad, it's likely to have as good or better a noise figure than the pre-amp.

[xmo]

In this application you would typically keep the receive frequencies within a fairly narrow spread - say not more than 1 to 1.5 MHz, use a window filter with steep skirts, then follow the filter with a preamp to provide sufficient gain to overcome the distribution losses.

The amp normally does not need to have a low noise figure at VHF - high 3rd order IP is more important. You can then insert the crystal filters after the multicoupler ahead of the station receivers.

There have been a number of large VHF systems installed recently [e.g. Virginia STARS]. This has helped raise awareness of the unique issues posed by the VHF RF environment. Many people are shocked at the amount of noise that exists at VHF and the degradation of effective receiver sensitivity that goes along with it.

Today's best practice for VHF system designs is to follow the noise and interference analysis methodology recommended in TSB-88.

Every site should be evaluated carefully as early in the system design process as possible. Noise levels should be measured and documented. When the system is installed noise levels should be measured again and compared to those used in the coverage prediction. Site logs should be created to record the effective sensitivity of each station at the site. Standardized procedures should be developed so that all techs know how to perform the tests on a regularly scheduled basis.

An abrupt increase in the site noise floor requires investigation and mitigation in order to preserve system performance.

[chartofmaryland]

Crystal filters add alot of insertion loss, on RX you will loose the gain of your preamp. I would put a quality notch filter on the TX and let the loss come from the TX side where it can go without.

The noise floor of the site needs to be measured at the three intervals so to map the performance you need to overcome the possibilities of interference.

I use exposed dipoles to keep noise down and use tower phased arrays when dealing with near site interference.

The choice I go with on high band in most cases is a tower top amp with preselector then bring it in the building through more pass cans where after I amp it one more time just to bring the users in the weeds up to 12db.

CoM