LNA oscillating or bias circuit not starting up

[avatarET]

Hi,

I am building a S-band to L-Band downconverter and my front end, mixer and LO is working great, however when I switch my IF amp (BFP740 amplifier design) on, the entire spectrum is full of spurs, MASSIVE ones. but if I touch the input or output port of the amplifier circuit, everything works as it is supposed to, I can take away the RF and re-apply it with no problem, but if I switch that amp of and on again the spurs come back. I have even made a little box around it for better grounding, no luck. I think the amplifier switches on and off the whole time untill you touch it.

Please help!

 

[BigBoss]

It oscillates or Power Supply is not well filtered.
Connect a 1K or more/less resistor at the output/input ( on the AC path).It will probably stop to oscillate.If it doesn't, check your filtering of power supply..

 

[chuckey]

If you measure the difference in frequency between the spurious outputs, this is the frequency at which it is oscillating. This will give you a clue as where your problem is. If its very low frequency then its bias or power supply, if its RF (IF) then its a RF decoupling problem.
Frank

 

[avatarET]

If you measure the difference in frequency between the spurious outputs, this is the frequency at which it is oscillating. This will give you a clue as where your problem is. If its very low frequency then its bias or power supply, if its RF (IF) then its a RF decoupling problem.
Frank

I have an ultra low noise LDO and filtering on the supply, decoupling is good, have a 100pF and 1nF everywhere necessary. The amplifier by itself works perfectly, its only when it gets the input signal from the LO that it does this

- - - Updated - - -

It oscillates or Power Supply is not well filtered.
Connect a 1K or more/less resistor at the output/input ( on the AC path).It will probably stop to oscillate.If it doesn't, check your filtering of power supply..

1K resistor to ground?

 

[chuckey]

Your IF amp is unstable, have you decoupled the Vcc and bias lines with at least a .1 MF +10 MF to stop LF feedback, is the amp DC coupled? Have you a proper IF filter preceeding it? and interstage screening? You still have not let on at what frequency its oscillating, depending on your IF frequency, a ferrite bead on any gate lead will help to stop VHF/ UHF gain and hence oscillations.
Frank

 

[BigBoss]

I have an ultra low noise LDO and filtering on the supply, decoupling is good, have a 100pF and 1nF everywhere necessary. The amplifier by itself works perfectly, its only when it gets the input signal from the LO that it does this

- - - Updated - - -



1K resistor to ground?

Yes, 1K ( or try the other values ) resistor to GND from Input or Output port.The position can vary so your should try some critical points on RF path.

 

[avatarET]

Your IF amp is unstable, have you decoupled the Vcc and bias lines with at least a .1 MF +10 MF to stop LF feedback, is the amp DC coupled? Have you a proper IF filter preceeding it? and interstage screening? You still have not let on at what frequency its oscillating, depending on your IF frequency, a ferrite bead on any gate lead will help to stop VHF/ UHF gain and hence oscillations.
Frank

Hi Frank,

Yes I have a 100 pF, 100nF and 1 uF on the supply, I have a very sharp BPF before and after the amplifier. looks like it is oscillating at around 1.4GHz, 1.8-2GHz and around 4GHz

- - - Updated - - -

okay, I have the amplifier running off a separate power supply and when starting up it draws half the current that it is supposed to, after touching it it draws the correct amount.

 

[tony_lth]

maybe it is your grounding issue, can you post your PCB and SCH?

 

[Bob60]

Hi,

(BFP740 amplifier design) on, the entire spectrum is full of spurs,

Please help!

It`s funny transistor. BJT SiGe with ft above 26GHz and Spar up to 20GHz and exist @ different packages and some of them are sens to a right VIA s and at right place. It is quite important. Otherwise you can get a lot of effects.