Changing GHz VCO frequency range | Frequency doubler at reduced frequency - Page 1
Forum section: electronics / rf-microwave

author: Amper
author_rank: Frequent Contributor
timestamp: March 21, 2019, 03:44:55 pm
content: Hi!Yesterday while roamig around on my local scrapyard i found two dozens of huges VSAT outdoor units. These are 13.75-14.5GHz trancievers to be mounted on sattellite dishes. One type is 1W, the other 2W and they have everything needed to build a nice little data transmission line or a 10GHz Ham beacon.After a few hours of work i think i understand most of the stuff inside and slightly modified one to transmit continuously without the signals for enable and power control. I just have to supply 15VDC and a signal between 100-120MHz to get a reading on the microwave counter.The issue i have is that i dont want to keep transmitting in the satellite relevant frequencies in Ku band for reasons of not wanting to pay hefty fines ^^The transmitter works by comparing the 110MHz signal coming from the indoor unit to a 1/64 divided version of the VCO output and doubling that roughly 7GHz signal later. Since 5.2GHz VCOs as a drop in replacement seem to be difficult to obtain i thought about modifying the one i have.It seems to be a simple one transistor oscillator with a varactor for frequency adjustment. Since im not a strip line expert that is where my understanding of the thing ends. If im correct and its just a varactor adding some fixed capacitance should do the trick in lowering the frequency by the needed ammount but randomly trying to poke around inside of the can did not change the reading whatsoever.Can anyone around here tell me if im wrong or did i just poke the wrong spots?Pictures of the entire board and the opened VCO are attached.cheers & Thanks in advanceA.

author: KJDS
author_rank: Super Contributor
timestamp: March 21, 2019, 09:20:38 pm
content: You'll also need to lengthen some of the lines.

author: asdf
author_rank: Newbie
timestamp: March 23, 2019, 11:12:50 pm
content: My guess is:- The transistor is a JFET, its source (6) is connected to ground through a biasing resistor (16) and an output attenuator (, its gate is connected to ground through a resistor (1); the output signal is taken from the source (6) through a transmission line (7), attenuator (and a coupling capacitor (21) which also removes some higher harmonics.- A varicap (20) is biased through a resistor (17)- The JFETs drain (5) is grounded for AC, the big pad (9) is a capacitor, the narrow quarter-wave line (10) is an inductor, the wide quarter-wave line (11) is a capacitor (transforms open to short), then another inductor (12) and capacitor (13) to filter power supply.- The source (6) is connected through an inductor (15) to a decoupling capacitor (14) which is also separated through another inductor (22) from biasing resistor (22) - the inductance may be used to make the transistor unstable and oscillating- The resonator is a series LC, where L is the wide (to reduce losses) line connected to the gate (21) and C is the varactor (20); the wide line connected to the diodes cathode (19) may be a capacitor that grounds the cathode or a some part of the LC circuit (inductive or capacitive, hard to say, it depends of frequency and size); the cathode is biased through inductor (18) and resistor (17)Changing the frequency may be difficult. Probably the transistor is used as negative resistance that is connected to LC circuit to compensate its losses (it's a common design approach but it's also possible that they designed it in a completely different way, taking advantage of various parasitics). The problem is that it may have proper negative resistance at the original frequency but not at others. Unfortunately it depends of the transistors parasitics and it's difficult to establish without a good simulation. If the negative resistance is not just enough and placed at the right frequency then the oscillation can not start, can oscillate at incorrect frequency (possibly not depending of varactor tuning), or exist at many frequencies (generating junk).It seems to me that possible ways to decrease frequency are:- to add a capacitor (probably <1pF) parallel to the varicap- to lengthen the line between transistors gate and diodes anode to increase its inductance- maybe modify the wide strip connected to cathode (hard to say what's its purpose but may be simple AC short to ground)- other lines (probably mostly the narrow line connected to the transistors source) can affect the transistors reflection but it's probably to hard to predict how.I'm beginner in RF so please mind that all/some of above may be incorrect.

author: Xnke
author_rank: Regular Contributor
timestamp: March 25, 2019, 06:28:01 pm
content: Flow a bit of solder onto the line marked 21, and see what happens. You'll be changing the characteristic impedance of the line, which will tend to detune the circuit and in effect make it appear "longer" without having to get in there wit the exacto-knife and cut castellations in each side of the line to physically lengthen it.

author: Amper
author_rank: Frequent Contributor
timestamp: March 28, 2019, 11:24:00 pm
content: Hi and thank you both a lot for your ideas!I will most likely buy a HMC530 and use it instead as it may work as a drop in replacement however i will still try modifications. Sadly recently killed this vco since it is the one in my "testing board" which was already mechanically and water damaged so i used it for reverse engineering of the control section. Since the Amplifier is still operational i will try to use the hmc530 here. As soon as i find the time for it ill open up a new transmitter and modify the vco as xnke told, making it thicker instead of longer is something i completely forgot about even though i have done it in the past on patch antennas, lets hope it works, this would be an amazingly easy thing to dooJust adding capacity did not turn out nicely at least in paralell to the varactor it just stopped oscillation, i guess 1p is still to much. I only need a few 10% anyways and i doubt this thing had 10pF in the beginning. Smaller caps i dont have and modifying strip lines seems to be the more elegant way here.@asdf I especially thank you for taking the time to write down all of this. Even if i will not achieve my goal here you still gave a very nice explanation and i learned a bunch of things by your words!Ill report when i know more, cheers A.EDIT:I may have to add that this might be a dark day as while repairing and re calibrating an old ft-7 ham transceiver the power went out and back again which reset the HP5350B 50Ohm to high impedance while having attached a x10 probe to the dummy load. I havent extensively checked my options yet but at least the low frequency input is history and if im even more unlucky i killed the 20GHz port as well. In this case this project will live in a cardboard box for quite a while...So as a PSA: Terminate 50Ohm by hardware outside of expensive devices, it may keep very sad things from happening to you...

author: Amper
author_rank: Frequent Contributor
timestamp: April 23, 2019, 05:38:14 pm
content: Hi!I finally got around to put the HMC430 on one of the boards. After removing the bulky old VCO there was enough space to just deadbug it to the microstrip, connect bias and control with wires and cover it with the old shielding can.So far it works beautifully regarding the generation of the 5-5.5GHz signal, the feedback loop seems to still work and i can control the frequency by the signal on the input of the module as it was intended.The worrying part s that the is not doing its job properly and since i dont know how it works i cant debug anything there...Is there anyone around here who has some resources explaining? I can not find a dedicated IC on the board, all parts in question have the same partnumber so it has to be implemented in stripline i guess.EDIT: The symptome im seeing is a ice 5G signal so the point of the first white transistor in the chain between the strip line and the discrete filter. After the transistor i can first see 5G, then something between 11 and 15G, but i cant pinpont the source. The doubler is in this area but i have no idea how such a wired signal can happen there.cheers

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: April 23, 2019, 07:16:16 pm
content: Quote from: Amper on March 28, 2019, 11:24:00 pmI have once tried to tune a ZCOMM VCO to lower frequencies by adding C in parallel to the varicap (and more varicaps) but I did not get very far. A little bit, yes. It was the usual architecture of a series resonator at the base of a capacitively loaded follower. The reason is clear. If you remove the resonator and measure into the base, you see a small C in series with maybe -50 Ohm.Adding much more C to the resonator won't help because it is still in series to the transistor's C.regards, Gerhard

author: Amper
author_rank: Frequent Contributor
timestamp: April 23, 2019, 07:39:15 pm
content: yes, it turns out its no as easy as i thought...maybe i will retry with the one i harvested today.Do you have any ideas about my new problem by any chance?

author: Amper
author_rank: Frequent Contributor
timestamp: April 23, 2019, 08:01:59 pm
content: mediocre neeeews everyone!After increasing the bias of the power stage and refitting the die cast shielding i got some 11GHz Output. Sadly only when holding the probe very very close to the waveguide almost putting it inside. Most likely before i just had so much stray stuff in all phases and frequencies that a precise measurement was not possible.This leaves two options in my understanding, either the filters are not passing enough to the power stage or the wave guide is to small. I guess now i need to learn more wave guide calculations a bit earlier then i expected.And more info:The wave guide is 19.1 by 7.5mm which should be good enough for my aim of 10GHz so i guess i need to add some cladding and reduce the filters frequencies...

author: Amper
author_rank: Frequent Contributor
timestamp: April 30, 2019, 01:56:08 pm
content: To those interested:Im continuing this Project here after i got stuff working by now.Thanks for all the help!https://www.eevblog.com/forum/rf-microwave/building-a-10ghz-band-ham-tranciever-from-hughes-vsat-hardware/msg2378592/#msg2378592