EMRFD Message Archive 7695
Message Date From Subject 7695 2012-08-27 21:53:01 Garry Hycas behaviour I have been testing the Hycas amplifier and have observed the following:
1. An AGC voltage > 4.5 Volts and the amplifier starts oscillating about 380Mhz
2. The gain of the first hycas stage is about 11dB
3. The gain of the second hycas stage is about 7dB
4. The gain of the third hycas stage is about 16dB
5. The total gain of the amp is only about 40dB.
My question is does the gain seem right? The QST article said the gain should be about 58dB. I built up a single stage of the amplifier with transformer output (for comparison purposes) and the gain is about 18dB. Therefore, using a 47uh inductor in the collector of the 2N3094 must lower the gain somewhat?
Has anyone observed number 1? If so any idea how to fix it?
7696 2012-08-28 10:29:11 Wes Re: Hycas behaviour Garry, et al,
I've not see VVHF/UHF oscillations with this circuit, although I have looked for them. The usual thing to do with a FET is to slip some resistance (50 to 100 Ohms or so) in the drain lead. Try some R in the collector lead. A small R in the gate lead might even help.
It is important that the bypass capacitor
7708 2012-08-30 03:42:23 ha5rxz Re: Hycas behaviour All agreed with what Wes wrote, an alternative to a resistor in the drain lead is a small ferrite bead on the gate lead. Some of the MMIC chips can oscillate at a couple of GHz and I cured that with a ferrite ring glued on top of the device, never did figure that one out.
7710 2012-08-30 08:08:03 Pete N6QW Re: Hycas behaviour Just to add to W7ZOI's comments --there are some really excellent Dual Gate MOSFETS now available --two in particular are the BF991 and BF994 available form NXP Phillips (Mouser & Digi-key). In single lots they are about 40 cents. For those who may want more info or suggestions on how to install the SMT versions of the Dual Gate MOSFETs see the following http://www.jessystems.com/DGM.html.
I have built a bilateral IF strip using two of the BF991 devices, one each side of a commercial 9.0 MHz filter and use the HYCAS AGC circuit for RX gain control. On Tx I switch the stages to fixed gain although this holds the possibility for ALC control. I have not seen any instability or oscillation problems with the HYCAS AGC. BTW I did bias the DGM stages in accord with W7ZOI's application note available on his website see (http://w7zoi.net/lna50.pdf)
One additional note I did sub a small trim pot and fixed resistor (total max R = 300 Ohms, min R = 200 Ohms) in place of the fixed "threshold' resistor. This enabled me to do some fine tuning on the AGC action.
In addition I have scratch built a second entire HYCAS module which is embedded in another XCVR and did not see or experience any instability or oscillati
7733 2012-09-03 19:24:45 Garry Re: Hycas behaviour Hi Peter,
I tried that link on your web site, but it didn't work
7734 2012-09-03 19:42:02 Garry Re: Hycas behaviour My apologies, if I click 7735 2012-09-03 21:19:24 RadiosRUs Re: Hycas behaviour I see the problem -- remove the period following the html http://www.jessystems.com/DGM.html
The above should work.
7739 2012-09-04 15:57:20 kb1gmx Re: Hycas behaviour 7743 2012-09-05 03:08:20 Garry Re: Hycas behaviour Hi Allison,
Thanks for your reply. I am testing at 9Mhz and I am using the circuit board from KA7EXM.
I have added an FB43-101 bead to the first stage fet drain and that has fixed the UHF oscillation.
As for the gain, I measured the overall output
7747 2012-09-05 16:40:31 kb1gmx Re: Hycas behaviour 7756 2012-09-08 08:45:35 kb1gmx Re: Hycas behaviour double comment...
One of the things I need to breadboard and try is the mixed device
cascode topology (HYCAS) using MOSFETS (bf170, 2n7000) as the input
FET or as the common base (or in this case gate) transistor of the
cascode pair. I do know the bipolar transistor and 12AX7 version works (kiksui O'scope vertical amp output stage).
No real applicati