Electronics > RF, Microwave, Ham Radio

High isolation LNA for the HF band direct conversion receiver

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szszjdb:
Dear All,

I am going to add a high isolation LNA before the common quadrature detector to protect the local signal leaking to the ANT port. But it seems few of the existing LNA design sample meet the requirement. The most famous elecraft KX3 use the dedicated isolation operational amplifier for that purpose which look like more expensive.  And the MCHF project just use the common emitter LNA ,which have poor reverse isolation. 
So I search around and find the cascode amplifier ,mostly in the modern 3-5Ghz LNA design might have much better reverse isolation performance than the above.  But few of the existing circuit or experience about the cascode amplifier in HF band application. Why? Any risk?

Does anyone could give more guide on the cascode amplifier in HF band application or the other information about the isolation LNA for the direct  conversion receiver?

Best Regards & 73!
James

UR5FFR:
Hi

Try simple common gate amplifier with J310. It have good reverse isolation.
If you need high gain you can use any amplifier before it. By example ERA MMIC.

A.Z.:
Hi there, I agree with UR5FFR about using a common gate JFET stage, it's simple and offers good input-output isolation, another idea may be starting with the tuned preamplifier circuit presented in the February 2018 issue of QEX and found here

http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb2018/Steber.pdf



the preamp is pretty simple/straightforward and offers good performances, willing to improve it, an idea may be adding a common gate JFET stage between the antenna and the tuned circuit input, such a stage should offer pretty LOW gain, it will only be used as a buffer to both isolate the antenna and offer a low input impedance suitable for feeding the stage using a standard coax; being the stage wideband, it will be better keeping the gain low to avoid overload from strong "off band" signals, the following varactor tuned circuit may be left as-is, or if aiming for multiband operations, modified to include a band switch to allow the circuit to operate over the whole desired frequencies range, the preamplifier may be modified too, if desired, an idea in such a case may be replacing the whole preamp (starting from R3 in the schematic) with an AD605 IC

https://www.analog.com/media/en/technical-documentation/data-sheets/AD605.pdf

such an IC offers a 40MHz bandwidth, which is more than enough to cover the whole HF range, plus it offers gain control, as is the gain control ranges from -14dB to +34dB, but adding a -10dB attenuator, it will be possible to bring the preamp to a -24dB to +24dB range which should be more than enough to deal with strong or weak signals, plus it would then be easy to add an AGC control to the AD605, in such a case the operator may still have a "gain" control knob, but it would just act on the max gain, while the effective gain (up to the max set by the operator) would then be controlled by the AGC

That being said, I'd avoid using a wideband RF preamplifier, in my opinion if you're seeking gain, better adding it where it's needed, that is after a tuned circuit, so that the preamp won't be overloaded by offband (off the DESIRED band) signals and offer much better performances

My 2 cents

[edit]

Forgot, if going to build the Steber tuned preamp (either using the original preamp or another one), it will be advisable to put a voltage regulator and some bypass capacitors on the line going to R2/VR1 to ensure the voltage driving the varactor will be stable and that it won't be modulated by RF leakage

David Hess:

--- Quote from: szszjdb on September 23, 2021, 05:53:00 am ---But few of the existing circuit or experience about the cascode amplifier in HF band application. Why? Any risk?
--- End quote ---

HF cascode amplifiers were more common in the past when transistor performance was worse and the most performance had to be extracted from each stage.  Cascode amplifiers are still found where ultra-broadband performance is required like cable systems and baseband applications like oscilloscopes.  Integrated circuit amplifiers of all kinds use them including many operational amplifiers.


--- Quote ---Does anyone could give more guide on the cascode amplifier in HF band application or the other information about the isolation LNA for the direct  conversion receiver?
--- End quote ---

A common base/gate stage provides the same benefit of high isolation.  Common base/gate HF RF amplifiers are common.

szszjdb:
Dear All,
ManY Thanks for your kindly help!

I have tried the common gate amplifier with J310 and found good isolation but gain dropping for about 6db when working on 6m band and above. The J310 were loaded with 200 Ohm and gain 6db on and below the 10m band. As for my current circuit,there have a bandpass filter in the front end ,following with the switchable 20db common emitter ampliifer, then the J310 and the quadrature detector. The sensitivity were poor above the 10m band due to the J310. So I am seeking for the more wider bandwidth circuit.

I will try the core of the Steber tuned preamp. The ADI/TI chip is so expensive now and I also heard it is a little noisy than the other type. I will try it later.

I found the BGA416 is the cascode type LNA and have 2 BJT inside it. So I built the similar one like the circuit below, but found poor gain when load with 50 Ohm in the output side.  The transformer in the circuit were replaced with just a inductor 10uH and the bias were 0.6mA. It seems the output impedence of the common base is far large than the 50 Ohm load. Still in tuning.

Any further comment are welcome ! Many Thanks!

Best Regards,
James

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