High isolation LNA for the HF band direct conversion receiver - Page 1
Forum section: electronics / rf-microwave

author: szszjdb
author_rank: Regular Contributor
timestamp: September 23, 2021, 05:53:00 am
content: 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

author: UR5FFR
author_rank: Contributor
timestamp: September 23, 2021, 10:31:16 am
content: HiTry 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.

author: A.Z.
author_rank: Super Contributor
timestamp: September 23, 2021, 01:41:03 pm
content: 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 herehttp://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb2018/Steber.pdfthe 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 IChttps://www.analog.com/media/en/technical-documentation/data-sheets/AD605.pdfsuch 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 AGCThat 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 performancesMy 2 centsForgot, 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

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2021, 03:07:03 pm
content: Quote from: szszjdb on September 23, 2021, 05:53:00 amHF 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.QuoteA common base/gate stage provides the same benefit of high isolation. Common base/gate HF RF amplifiers are common.

author: szszjdb
author_rank: Regular Contributor
timestamp: September 23, 2021, 05:37:56 pm
content: 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

author: A.Z.
author_rank: Super Contributor
timestamp: September 23, 2021, 05:45:09 pm
content: Hi James, the common gate (or common base) amplifier isn't generally used for gain, but to offer good I/O isolation, I will repeat it, if you are seeking for gain AND I/O isolation, a wideband, untuned common gate stage, followed by a tuned stage, which in turn feeds a good gain preamp is the way to go imVVHo, and I believe that the simple, yet effective preamp designed by Steber may be a good start

author: szszjdb
author_rank: Regular Contributor
timestamp: September 23, 2021, 06:13:00 pm
content: Hi,A.Z.,Many Thanks!Agree with you that the J310 is just for isolation. But it will introduce 6db or more gain drop on the 6m band and above, which make it hard to compensate for it.Does you mean to change the cascade order to improve the gain loss in the higher band ? Move the J310 to right behind the frontend BPF instead of before the quadrature detector stage?Best Regards,James

author: A.Z.
author_rank: Super Contributor
timestamp: September 23, 2021, 06:21:32 pm
content: Quote from: szszjdb on September 23, 2021, 06:13:00 pmHi there again, James, are you sure that the gain decrease (loss if you want) isn't due to the common gate/base design you are using ? A schematic would greatly help.

author: szszjdb
author_rank: Regular Contributor
timestamp: September 23, 2021, 07:14:24 pm
content: Hi,A.Z.,Many Thanks!The circuit is as attached. And I could find the voltage output of the J310 decreased to the half of the voltage on 10m band ,when switch to 6m band and above. The MDS test also prove that gain drop. The transformer were proven to be 0.3db loss below the 150Mhz range. It seems that the gain loss might introduce by the mismatch of the impedence between J310 and the following quadrature detector FST3253 , but have no idea to compensate for it.Best Regards,James

author: UR5FFR
author_rank: Contributor
timestamp: September 23, 2021, 07:47:07 pm
content: HiCommon gate cascade have gain depended from |Z| load. Are you sure that your quadrature detector on FST3253 have same input impedance in range 2..50MHz? I think not have.Try to insert simple emitter follower between J310 and detector. And place resistor with 100-200ohm parallel to inductor L51 in j310 drain.

author: KE5FX
author_rank: Super Contributor
timestamp: September 23, 2021, 10:41:55 pm
content: A J310 should still be capable of providing good performance at 6 meters. Try supplying the drain voltage through the transformer primary itself, rather than the 47 uH choke. It may have enough parasitic capacitance to hose you at 50 MHz.

author: szszjdb
author_rank: Regular Contributor
timestamp: September 24, 2021, 07:34:52 am
content: Hi,UR5FFR & KE5FX,Many Thanks!It's great! I will test and feedback to you.Update:1. Remove the choke and connect the J310 dirrectly to the transformer, a little improvement in the j310 drain voltage.2. Add 470ohm parallel to inductor L51 in j310 drain, disconnect the transformer ,no adding the follower, still found 6db gain drop on the drain voltage comparing to 20m band and 3db to 10m band.It seems common gate J310 have no voltage gain on 6m ,3db on 10m and 6db on 20m band. That is really a headache.Any further advice are welcome.Best Regards & 73!James

author: UR5FFR
author_rank: Contributor
timestamp: September 24, 2021, 08:20:01 pm
content: HiJ310 very well working on high frequency. If you open datasheet you can find that NF specified for 450MHz.My suggestion still unchanged - insert emitter follower between J310 and detector

author: A.Z.
author_rank: Super Contributor
timestamp: September 26, 2021, 09:39:07 am
content: my guess is that the input and output circuitry is cutting gain when going up in frequency, I'd try modifying the values of c207/c218, l52, r83 and eventually c202, notice that the input impedance of the j310 in common gate config is around 70 Ohm (if memory serves me correctly)

author: szszjdb
author_rank: Regular Contributor
timestamp: September 26, 2021, 06:06:36 pm
content: Hi,UR5FFR & A.Z. ,Many Thanks!I found out the gain drop is due to the ceramic inductor L51/L52. Replace them with the SMD Wire Wound Chip one, the gain recovered.The isolation is around -30db, reasonable for such simple circuit.Best Regards & 73!James

author: A.Z.
author_rank: Super Contributor
timestamp: September 26, 2021, 06:46:51 pm
content: Happy to know you found a solution, James; now I'm curious, are you building a receiver ?

author: A.Z.
author_rank: Super Contributor
timestamp: September 26, 2021, 07:27:39 pm
content: forgot, if you're still seeking for ideas, have a look at thishttps://www.qsl.net/g3oou/highdynamicranferfamplifiers.html

author: szszjdb
author_rank: Regular Contributor
timestamp: September 27, 2021, 01:07:11 pm
content: Hi, A.Z. ,Many Thanks for your kindly help!Yes I am designing in my own transciever in the ZIF archetechture. In the RX side, it is include the BPF , LNA , isolation , QSD and the audio ADC. I am focusing on the frontend these days. Now is the LNA. I am trying the common emmiter amplifier as attached, but not satisfied with the high power comsumption. Actually , the following quadrature detector and the ADC will saturate arround -5 dbm. I am wondering if I need so high IP3 amplifier or just like the KX3 to decrease the Vcc to 3-5V to minimise the current drain. Or just using some up to date integrated LNA , like BGA416 , only 5 mA.Would like to have your advice!Best Regards & 73!James

author: A.Z.
author_rank: Super Contributor
timestamp: September 27, 2021, 01:41:59 pm
content: Hi James, thanks for satisfying my curiosity !As for the preamp, that W7IUV is a "classic" (checkthis) but it has been designed for optimal performances in the 160 to 40 meters bands, given that you're aiming to go up to the 6m band, I wonder how well it may work "up there", also consider that if the plan is putting a wideband, untuned preamp at the receiver antenna input, then I believe you'd better revise your approach; in my very humble opinion, it would be a better idea placing a low gain stage there, and a stage capable of both preamplifying the signal a bit (say +6dB or the like) or attenuating it (say -10dB or more), following such a stage with the preselector (bandpass) for the given band and after the preselector adding another preamp with the desired max gain, such a second preamplifier may also (optionally) include a tuned circuit (like the "Steber" one - see my previous post) with the ability to bypass the tuned stage; that stage, if inserted, may allow to further narrow the input preselector and improve the input signal; that way you'll avoid issues from IMD/Overload caused by strong offband signals since the gain will only come AFTER the preselector and the input stage will anyways allow to both recover the loss from the bandpass filter and attenuate strong signals (if needed) to avoid overloading the input stage (and the preamp sitting after the preselector); anyhow, for other preamp ideas, have a lookhere, another (very simple, don't expect stellar performance) input circuit may be the one in the attached imagefurther preamp ideas may also be foundhereMy 2 cents.

author: UR5FFR
author_rank: Contributor
timestamp: September 27, 2021, 07:59:14 pm
content: Hi JamesAmplifier with collector-base negative feedback have poor isolation. It input impedance depended from load (see EMRFD and other). Any MMIC have very similar topology andneed50R load. Good choice for isolation amplifier is common gate or common emitter with emitter-base negative feedback.You can find more different amplifier design on my sitehere

author: KE5FX
author_rank: Super Contributor
timestamp: September 27, 2021, 10:03:50 pm
content: Quote from: UR5FFR on September 27, 2021, 07:59:14 pmVery nice site, Andrey -- I like the way you use the message board to organize your personal notes.(Edit: I added a link to your page, via Google Translate, at the bottom of my "Sort-of-but-not-really-a-Norton amplifier" pagehere.)

author: David Hess
author_rank: Super Contributor
timestamp: September 27, 2021, 11:50:19 pm
content: Usually I think of two transistor cascode amplifier for high isolation, and they are used for exactly this in precision timing applications, but while I was searching, I found no examples of *reverse* cascode RF amplifiers. Is this because they rely on complementary transistors?Update: I should have written folded cascode instead of reverse cascode.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 28, 2021, 08:08:02 am
content: I have made an isolation amplifier as a replacement for a NIST circuit that usedRF PNPs which are a dieing-out species. OK, I still have a reel of BFG31 :-)I'm still not happy with it; backward isolation is only 80 dB @ 100 MHz andS22 is a mess. The input tantalums are to short the bias voltage noise throughthe low impedance source. The tantalums in the bias chain make a differencein isolation.Reverse isolation is 130 dB @ 5 MHz.On the UR5FFR web site I have seen the Rohde amplifier with emitter feedback.For me, that was a power oscillator at least with BFQ19S and MACOM transformers.Gerhard DK4XP

author: KE5FX
author_rank: Super Contributor
timestamp: September 28, 2021, 08:38:00 am
content: What does the reverse isolation do when you disconnect the power? It should get worse.What about when you remove the metal shield? Obviously enough, it should get worse then, too.If doing something that should make the isolation worse makes it better instead, that's always a good clue. Sometimes trying to make the problem worse is the best troubleshooting strategy of all.There's a good ground plane on the other side of that board, right?What bad things happen if you install a cap bigger than 10000 pF at C10?

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 28, 2021, 09:14:17 am
content: Backward isolation is not really a problem at 130 dB?I had to use 1 Hz resolution bandwidth on the VNA to see it. :-)Top & bottom of the board should be quite rf-tight. My home-etchedboards have usually an unbroken bottom ground. That makesaligning the masks so much easier.The weak point is probably the bias string. The 100u tantalumsare a great step forward. 680uF has no added value.Increasing the C10 capacitor too much seems to introducebias instability.That looks ugly. I should wash the flux off.

author: iMo
author_rank: Super Contributor
timestamp: September 28, 2021, 09:33:02 am
content: The FST3253 mixer has high IP3 thus it would require something more elaborate in front of it (than a single jfet), imho..PS: Norton feedback preamp?

author: KE5FX
author_rank: Super Contributor
timestamp: September 28, 2021, 09:44:35 am
content: Quote from: Gerhard_dk4xp on September 28, 2021, 09:14:17 amAh, I thought you were complaining that it wasn't good enough (presumably at 100 MHz.)80 dB at 100 MHz isn't bad at all, depending on what you need it for. You won't have injection-locking problems at that level, certainly.

author: UR5FFR
author_rank: Contributor
timestamp: September 28, 2021, 10:17:05 am
content: HiQuote from: imo on September 28, 2021, 09:33:02 amYou right - for FST/FSA based mixer preamplifier must be have high IP3. By example 4*J310 with total current 80-100mA or BFG541/BFG591 with current 70-100mA.

author: iMo
author_rank: Super Contributor
timestamp: September 28, 2021, 10:18:27 am
content: Norton-feedback one? Not sure on isolation, but for HF you do not need much, imho..Some measurements:https://www.kn5l.net/rf-amp/norton/https://www.okdxf.eu/lankford/Noiseless%20Feedback%20Norton%20Amplifiers.pdf

author: UR5FFR
author_rank: Contributor
timestamp: September 28, 2021, 11:43:31 am
content: Norton-feedback have poor isolation. It input impedance depended from the load

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 28, 2021, 06:03:46 pm
content: LTspice sim added for completeness.An RF preamp is probably not needed on short wave for sensitivity, more to reduce osc emissions.Much more important is proper termination of the RF and IF ports of the mixer.Everything that is reflected into the mixer undergoes a new cycle of mixing,creating higher order IMD in the process.V3 does nothing interesting; it just prevents LTspice to do a divide by zero or sth. like that.60 Ohms at room temp. delivers 1 nV/rt Hz thermal noise. Unimportant in the time domain.

author: szszjdb
author_rank: Regular Contributor
timestamp: September 28, 2021, 07:06:29 pm
content: Hi,dk4xp,Thanks a lot!It is great！ That is the multi-stage cascode circuit. How about the current drain and the output impedence? The common base seems a little high output impedence.Best Regards,James

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 28, 2021, 07:23:01 pm
content: The collector current varies between 5 and 125 mA or so, as you can see in the red traceof the upper plot pane. The idle Icc is about 65 mA. This is for 13 dBm max. output.If you accept 10 dBm or even less you can reduce that by increasing R8. Same whenusing a 4:1 output transformer. But the transformers I had in the drawer gave weird S22.For 13 dBm, the BFQ19S with its 120 mA Ic abs max is just so enough. BFQ790 should be comfy.Here the transistors behave as an ideal current source and the 50 Ohm resistor dictatesZout. the 50 Ohm resistor at the right is the simulated load.Q2 is a voltage-to-current converter; The emitter resistor is the scale factor.The PNP only adjusts the operating point.This was intended as one channel of a distribution amplifier. You can parallel someinputs that are medium impedance and adjust Zin to 50 Ohm with a resistor.

author: KE5FX
author_rank: Super Contributor
timestamp: September 28, 2021, 09:47:12 pm
content: The series combination of D1 and D2 is interesting. Those low-voltage (true) Zener diodes are next to worthless in my experience; do you actually find that they are doing anything at all?Seems like a 6.1 volt Zener (i.e., avalanche) diode would do better, but I guess the reasoning is that it would be noisier. Maybe a couple of blue LEDs would work?

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 29, 2021, 03:59:50 am
content: The true Zeners work. Yes, they are softer. But it is not important if it is 100 mV more or less.Maybe it would work without the diodes but I wanted less unknowns / easier changeswhen debugging the bias system.One of the pictures is the noise of Zener diodes. One can easily see the strong riseof the noise vs. rise of Vz. Between 2V7 and 6V are worlds, in the same series.Diodes are tested with a 1k or 2k wire resistor from 10 NiCd cells.The NiCds add no visible noise.The steep rise with 1/f**3 on the left side of the plots goes on the measuringamplifier resp. its input coupling capacitor. 6 years ago, that was 100 uF foil,not enough by far. The short & 60 Ohms are on the amplifier side of the capand show what the amplifier can do really with a low impedance DUT.A 4700 uF wet slug tantal healed that. The input stage of the preamp consists of20 ADA4898 op amps (10 pairs) in par and averaged into an inverter.Noise in the flat part is abt. 220 pV/rt Hz . The 0 dB line is 1 nV/rt HZ.I now have an amplifier based on 16 CPH3910 FETs that is nearly in the sameleague but with less noise current and a reasonable input cap. I must findthe time time for the write-up. The smaller noise current also opens theway to cross correlation with the Agilent 89441A. The effect of the noise currentis common mode in the DUT and would not average away.Among LEDs, blue ones are bad. King of LEDs is HP/Agilent/AvagoHLMP-6000 if you want low noise. Otherwise, it's quite a dim bulb.There is also a LT3042 for comparison.For completeness, there are also some regulators.The LT3042 is standing out.The LM329 is from Digikey, marked NS, years after TI bought NS.It does not look like a Zener, more like a bad bandgap.

author: KE5FX
author_rank: Super Contributor
timestamp: September 29, 2021, 04:17:06 am
content: Good stuff. I have a bunch of CPH3910s in the drawer, as well as an 89441A that could sure use a quiet preamp, so will keep an eye out for more details on that.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 29, 2021, 04:33:21 am
content: The traces are taken with the 89441A. Since it cannot make nice pictures overseveral decades, I have written a program to control it via thin Ethernet / GPIB andmake measurements over ~ 7 decades. Then the results are plotted with Gnuplot.I never got GPIB working completely with that yellow USB/GPIB dongle.I found out that I have to do a seek if I switch the direction of the socket interface,even if not wanted, and newer Linux kernels got more strict about that from releaseto release. After that there was no more point in using GPIB.

author: szszjdb
author_rank: Regular Contributor
timestamp: September 29, 2021, 08:52:12 am
content: Dear All,I have another question about the LNA.How to determine the proper MAX 1 db compression power output for the LNA in the HF band receiver , given that the quadrature detector and the following ADC saturation point at -5 dBm ?The bigger should have better OIP3 performance but more current drain. It should also consider about the in band and out band interference ,especially the AM BC signal. The famous KX3 use only a common emitter amplifier with the P1dB for about +5 dBm and 10 mA bias. That is enough?Best Regards,James

author: iMo
author_rank: Super Contributor
timestamp: September 29, 2021, 09:22:26 am
content: It is not only matter of IP3 but also of input band pass filtering.

author: A.Z.
author_rank: Super Contributor
timestamp: September 29, 2021, 09:27:25 am
content: Quote from: szszjdb on September 29, 2021, 08:52:12 amJames, I already wrote it and I'll repeat it again; don't put a wideband preamp with high gain directly at the antenna input; put a (set of) bandpass filter(s) at the antenna and connect your preamp after the filter, this way the preamp will only work on the portion of frequencies of interest, also ensure to properly adapt the input and output impedance of the bandpass so that it will work properly, a simple trick to do that (but there are a number of other ways), assuming the antenna input has an impedance of 50 or 75 Ohms, could be adding a simple common gate stage (again) configured for LOW gain at the antenna input, the output of this stage will go to the bandbass and the output of the bandpass to the preamp, the preamp should present an input impedance similar to the one at the output of the common gate and the bandpass should be designed to work at such impedance

author: szszjdb
author_rank: Regular Contributor
timestamp: September 29, 2021, 09:58:35 am
content: Hi, imo and A.Z.,Many Thanks!Yes there have a bandpass before the common gate stage and the switchable preamp in my design. In the normal condition of lower HF band, there is no need for the preamp, and the IP3 performance is determined by the low gain common gate and the QSD itself. That should be no problem. But in the higher aband like 15m to 6m , there will often need that preamp. The out band signal I mentioned means the band nearby. Due to the wide span of the bandpass filter , e.g. the 20m's is from 8-16Mhz , the 30m signal will come with the 20m band when preamp is on. So I am considering the proper P1dB or the OIP3 performance to just meet the requirement of the following QSD stage.Best Regards,James

author: A.Z.
author_rank: Super Contributor
timestamp: September 29, 2021, 10:20:22 am
content: Quote from: szszjdb on September 29, 2021, 09:58:35 amyour bandpass is too wide, as I see it (hope others will correct me if I'm wrong), you have two possible solutions; one is use narrower filters, one for each band of interest, another (but may be mixed with the first one) is using tunable filters so that a single filter may cover a given range but still have decent bandwidth to cutoff the adjacent bands

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 29, 2021, 11:11:59 am
content: Back when I found shortwaves interesting, I built a filter that cutthe 80m band into 4 ranges.5 Siemens K1 pot cores with Q> 400, reed relays and mechanical trimmers.Sorry, can't find it for a photo. :-)

author: UR5FFR
author_rank: Contributor
timestamp: September 29, 2021, 11:26:07 am
content: HiI can add some important thing about BPF for direct conversion receiver. You need suppress 3F and 5F frequency as much as you can. Key mixer converts on odd LO harmonics. On 3F frequency it have -10dBm conversion gain! This is mirror channel for DC receiver.So when you choose BPF check it in simulator (by example RFSimm99)

author: A.Z.
author_rank: Super Contributor
timestamp: September 29, 2021, 12:41:15 pm
content: Quote from: Gerhard_dk4xp on September 29, 2021, 11:11:59 amI think that it was a pretty interesting thingGetting back on topic, if you look at my earlier post about that "Steber" tuned amplifier, such a thing may help solving the issue, what I mean is; let's say that the OP has a 20m band bandpass filter with a range going from 8 to 16 MHz, now, if we add the simple varactor tuned (or a better one btw) circuit in front of (or right after) the bandpass and adjust it following the main tuning...

author: szszjdb
author_rank: Regular Contributor
timestamp: September 29, 2021, 03:48:54 pm
content: Hi, A.Z., dk4xp and UR5FFR,Many Thanks!The existing 30-20m bandpass have a 3dB bandwidth from 9.5 to 16.9Mhz and -40dB at 30Mhz and -60dB at 42Mhz. I will fine tune the filter and there might have some trade off as I am designing a portable QRP unit. As running by the battery, the current drain is critical.Then how about the P1dB or OIP3 requirement for the following preamp? Does a P1dB +5 dBm with 15-20dB gain preamp fulfill to the QSD with -5dBm saturation point?Best Regards & 73,James

author: A.Z.
author_rank: Super Contributor
timestamp: September 29, 2021, 04:33:15 pm
content: forgive me, but I can't see how a rx bandpass filter would have an impact on battery draina set of bandpass filters, one for each ham band won't require too much components (L/C) and will help avoiding out of band issues

author: szszjdb
author_rank: Regular Contributor
timestamp: September 29, 2021, 05:09:19 pm
content: Hi, A.Z.,Sorry, I mean the preamp. The higher OIP3 the higher current drain and the Vcc. I just want to make sure the bias point of the preamp is just fulfill to the system requirement. What should be the banlance point?I am considring refining the bandpass filter in the very front end.Best Regards,James

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: September 29, 2021, 05:35:02 pm
content: Some pointers:Really about noise blankers, but that's the same area in the receiver:<https://dk4sx.darc.de/eigenbau_rx.htm><http://www.oe3hkl.com/rx-homemade/rx-mit-h-mode-mixer-ip3-45dbm/stoeraustaster.html><https://worldradiohistory.com/Archive-DX/Ham%20Radio/80s/Ham-Radio-198006.pdf><https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1994/05/page29/index.html><https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1994/06/page27/index.html><https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1994/07/page42/index.html>There is a lot more on Rob's web IIRC.S9 is defined as 50uV in the 1/3 Rohde articles. 0 dBm is 635 mVS9 = 50 uV is -73 dBm for comparison.73, Gerhard

author: szszjdb
author_rank: Regular Contributor
timestamp: September 30, 2021, 02:54:44 pm
content: Hi, dk4xp,Many Thanks!Your information help much!It seems the preamp P1dB should have 3 dB higher than the QSD and following . As the saturation point of QSD is about -5 dBm, the 0 dBm P1dB of preamp should be enough for my system. Is that right?Best Regards & 73,James

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: October 06, 2021, 08:21:33 am
content: ?? I do not understand the question.In the mean time I have found a pic of the preselector.

author: szszjdb
author_rank: Regular Contributor
timestamp: October 08, 2021, 05:32:57 pm
content: Hi, dk4xp,Many Thanks!I just want to make sure the linear output level of LNA is within the margin of the following quadrature detector and the ADC. If the P1dB of the LNA is too low , the 2nd and 3rd distortion component might degrade the IIP3 performance. If it is too high, more current drain.Best Regards & 73,James

author: A.Z.
author_rank: Super Contributor
timestamp: October 08, 2021, 06:01:20 pm
content: I can't still understand if you are on some kind of assigment or whatever, not willing to give offence, but sincerely... at times you seem to be a skilled EE and the second later you look like a total newbie, now, while there's nothing wrong with that (ok almost), it seems like you start by reading a question from someone else and then get down to being cluessless, again not willing to offend, but sincerely, to me, it doesn't look like the kind of "walk the talk" kind of behaviour, then, by the way, I'm just another newbie here, so I may be totally wrong

author: szszjdb
author_rank: Regular Contributor
timestamp: October 09, 2021, 02:51:39 am
content: Hi, A.Z.,Many thanks for your help!Yes I am not familiar with the LNA and willing to have more advice on the design. Sorry to make you confuse.From the testing these day, I found many cross modulation and intermodulation distortion when I switch on the LNA in the 10m band in the afternoon. It seems a lot of modification needed, the BPF and the LNA.Best Regards & 73,James