[Collins] Interesting snippet on today's Collins - MiniCircuits Labs

[Dr. Gerald N. Johnson]

On 4/28/2010 1:50 PM, Carl wrote:
> Im sure some radios need all the help they can get. In my case I use
> modified TS-830's as 28 mHz IF's for all the transverters so gain isnt
> an issue.

The 736 is deficient in front end gain, then most users apply an 
external preamp and hill of what top end it had.
>
> Im about 1 1/2 miles LOS on the next hilltop (My ASL is a bit higher)
> from a comm site with UHF TV, FMBC stations, all sorts of 2 way, paging,
> government and probably other things generating all sorts of IM right on
> site. To add to the mess a commercial 2 way site is at the house across
> the street with about 10 VHF/UHF sticks on a 150' tower.

I'm moving further and higher.
>
> I come thru RX BPF's from 6M up thru 1296 and try to suffer minimum NF
> degradation while having a crud free receiving enviroment. Luckily for
> the guy across the street Im almost never aiming the 1500W fed 2M array
> at him as the few times I have has folded up his High Band VHF radios
> which include the towns police and fire.
>
> DBM's can have the IF port with a terminated image frequency idler
> circuit and still maintain excellent IMD and impedance. The RF amp sets
> the system NF with no more gain than necessary and the narrow band IF
> amp then takes care of any LC loss.

yup.
>
> Im a firm believer of gain distribution to maximize performance.

Has to start with the gains all maximized, and all have to be adjusted. 
Determine the system NF with the gains maximized, then start at the IF 
selectivity and start lowering that stage gain until the NF changes, 
then increase that stage gain just enough that the system NF is 
unchanged. Then move a stage forward and repeat. Sometimes it takes 
attenuators between modules to get the gain down, but the procedure is 
most effective if done by the stage. I learned that by experiment in 
1964 or 1965. Collins S line receiver with nuvistor converter. I was 
living in Allen Tx at the time and W5WXV in Garland with our antennas 
pointed at each other was 18 or 20 places in the first 200 KHz of the 
band (no CW sub band then) while transmitting very narrow FM on 144.085. 
When I got done (starting by minimizing noise contributions of the S-3B 
stages from the filters forward, and that included a magnetic shield 
over the mechanical filter) and included a fixed attenuator between the 
converter and the receiver I had the same system NF but W5WXV was only 
three places in that first 200 Khz. Full scale on .085 and at the noise 
level the other two places. A great improvement.

With lossy mixers involved, it might take a couple sweeps when the front 
end gain is excessive and making up for mixer loss too much.
>
> Carl
> KM1H
>
The double balanced mixer in the FT-736 on 2m and 220 is coupled 
directly to the crystal filter so there's no image or off frequency 
matching there.

73, Jerry, K0CQ