Configuring Pre-amps to Your Board
There are a few areas regarding the Construction, Assembly and Alignment that I believe are worth commenting on before putting everything into practice. One, and perhaps MOST important is the input line. You will note that I mention a specific “N” Male plug ……. the Male plug MUST have a captivated Pin, regardless of who the Manufacture and what the Part Number is …. a MUST believe me.
Secondly, if you can NOT use the specific “N” fitting mentioned, then Mechanical alterations to the Brass LIne will have to be made to maintain the Line Impedance ….. AND insure secure holding of the Captivated Pin Assembly. For example I have used “N” Male fitting that are Captivated Pin ….. but DO NOT require the shallow sholder to clamp the twoTeflon rings in place. If that should be the case …… simply mill that shallow sholder OFF … in nearly all cases that will be all the is required. The Nut holding everything down will, in some cases, have to be drilled to 24.5mm …. but NOT larger, or it will jam OVER the Brass securing shoulder of the fitting.
Thirdly there should be NO additional Teflon ADDED anywhere EXCEPT the very narrow 1 to 1.2mm thick piece at the coupling end of the Line, i.e. It MUST remain essentially an AIr Dialectic Line to acquire the desired N/f of approximate. 0.2db. The center conductor of the input line is polished Silver Rod. …….. Brass will work, but it MUST be polished and very clean for 1.3Ghz and maintain the same Dia. as the Pin body. Anyone wishing Silver Rod for their project please contact me …… I have several meters of it.
After everything is assembled, I use 2 DVM’s one to measure voltage drop across the 220R resistor, and one to measure the Drain to Source Voltage ….. solder a small piece of wire to one Leadless Disk Cap. to attach DVM lead. Before attaching the 27R to the Drain …… preset the voltage regulator to approx. 4VDC output and the Source Trimmer to one half (50%) range. Now attach the 27R to the Drain.
Using all 4 hands adjust the Regulator and Source Trimmer for ~3 to 3.5 DCV Drain to Source and approx. 10 to 12mA Drain current, or 2.6VDC across 220R ….. this is NOT critical but get close.
After everything is connected to your N/f Meter and resonated to 1296Mhz … check the N/f ……. it will most likely be HIGH if you installed a 25mm length of #32 wire to the Gate ………. using small nippy cutters start clipping the #32 wire ~1mm at a time until you reach minimum N/f. My experience is that no 2 devices are exactly the same for this clipping adjustment, or Drain current, and Drain to Source voltage, these last two parameters are the least critical …. so tweak until you are happy you are optimized for best N/f
AGAIN Folks …. NO two devices will, unless you are lucky, perform identically the same, but they will be very, very close and N/f will always be in the area of 0.2 to 0.25db. Applied input voltage at the FT Cap. can be anything between 8 and 38VDC, (THIS will be determined by the voltage rating of any Tant. or like capacitor installed . i.e. I have one rated at 25VDC ….therefore 24 is highest voltage), and hang filtering any where and a s much as is convenient to place …. but keep area CLEAN of components around the active device.
One closing comment on Stability. There was one device that acted a bit odd when switching between TX & RX … only at times …. it turned out to be a slight bit on the unstable side …. adding a small piece of RF Absorber as shown in one Image above cured that problem completely. Sample Kits of RF Absorbing Material are available from Emerson Cuming Company as LINKED above. 60% of the sample you will possibly NEVER use, but the price is right. Instability can also be cured in most cases by tighting the input coupling …….b ut N/f will suffer some what.
I noticed very little, if any difference between Silver Plated Cavity’s and non-plated cavity at 1.3Ghz …… but material MUST be clean and polished then cleaned of ANY & ALL residue completely with 100% Alcohol. DO NOT DRINK, Pre-Amp becomes fuzzy in appearence and rather distorted, and in fact may KILL you.