LF Spectrum Analyser

Any recommendations for a spectrum analyser, say =1MHz, maybe USB, hopefully inexpensive, definitely available now, in the UK?

Aaronia any good? Not generally very good reviews, but people here may know better.

Cheers

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Clive
Reply to
Clive Arthur
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Am 27.06.2017 um 22:25 schrieb Clive Arthur:

A used Agilent 89441A. The IF unit would be enough probably.

Available cheaply because people think it is usable only for checking 3G cell phones.

cheers, Gerhard

Reply to
Gerhard Hoffmann

Just playing around with one. Performance about ok for my purpose (generator magnet field EMC measurements), but the software feels uncertain at least on Mac. As an example, I'm having hard time to keep the unit as dBuV while changing span or some other parameters.

I'll compare with HP 3585A when I get back from the gig.

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mikko
Reply to
Mikko OH2HVJ

What's low cost? I've been lusting after a Rigol SA. I see there is now a low end (100 kHz to 500 MHz) one ~ $700.00, but I would like a DSA815 with TG. There seems to be a divide in the SA market at ~100 kHz. With 'audio' SA's from milli-Hertz to 100kHz, and then the RF stuff.

I use the FFT on my 'scope as a low end SA, but it's not very good at zooming in on one limited frequency window. What are you doing?

George H.

Reply to
George Herold

Do these 100 kHz 'audio' SAs have proper S&H at input ?

In that case use a local oscillator (LO) to mix up a frequency segment of interest to a convenient intermediate frequency say 45 MHz. Using a

100 kHz roofing filter to limit the spectrum to 45.0 to 45.1 MHz and feed it to the 100 kHz audio SA.

To select a different 100 kHz segment,just step he LO in 100 kHz steps. Due to the big frequency step, even a PLL can be used as the LO, without introducing too much phase noise.

Search for "waterfall display" and WebSDR. These often use an I/Q (Zero IF) front end with cheap "audio" A/D converters.

Reply to
upsidedown

I have a Signalhound.

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Supposedly it goes down to 1Hz but I never used it much below 150kHz. So far it has found all the stuff I was looking for but you have to keep in mind that these budget-level analyzers are in essence glorified software-defined radios (SDR). They convert down and analyze the spctrum in slivers, then piece together the results in software. This has two disadvantages, usually no good image rejection (it's done in software) and they cannot handle pulsed signals very well. The main reason I bought it was its portability.

Also, be very careful with the input amplitude because at 10Hz you can't reasonably protect that by AC coupling.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Sample and Hold? I don't think so. We've got an SRS770. I've done the mixing trick to look at somewhat higher frequencies. (a few MHz.) But not the best filtering, so all sorts of aliasing (if that's the right term) of stuff into the 100 kHz window. (I'd figure out what is what by changing the LO and seeing how things moved.. not the best.)

George H.

Reply to
George Herold

Any experience of this...

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14 bit ADC, inexpensive, would need hiding in a box. Maybe even buy a cheap Chinese USB scope just for the enclosure.

Cheers

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Clive
Reply to
Clive Arthur

The reason that I was asking is that while in traditional ADCs with S&H can easily handle _bandlimited_ high frequency signal by decimation, the only critical parameter is _transition_ time between sample and hold state. Thus, if the transition time is about 1 ns, quite high frequencies can be sampled without too much aperture error from say a band limited 50 MHz IF signal.

But how about delta/sigma converters, can they handle bandlimited high frequency signals and properly decimate or is a S&H stage needed in between ?

Using some common intermediate frequencies, such as 455 kHz, 10.7 MHz or 45 MHz and there is a huge number of commercial ceramic or crystal filters with a few kHz to at least 300 kHz bandwidth available. These filters will help getting away with most of the aliasis.

For a more modern approach, take a look at

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and try out some of these receiving station. These typically digitize a whole amateur radio band, producing a spectrum and waterfall display.

In addition, several clients over the internet can connect to this station, independently tune around the digitized band, perform demodulation (CW, SSB, AM or FM) and transfer the audio over the internet to individual clients. Of course SSB "demodulation" is just a frequency transfer, so the end user could do any analysis to that "audio" bandwidth.

Most of those WebSDR stations are simple PCs with a stereo 192 kHz sampling rate sound card. Some use I/Q mixers driving the left and right audio channel. The local oscillator can be a sin/cos DDS or simply a crystal with 90 phase shifter.

All the rest is done in PC software.

Try one of those.

I still do not understand how somebody can sell 100 kHz SAs (SRS770) starting from USD5000 :-)

Reply to
upsidedown

OK Thanks... I don't know anything about SDR. (except in theory) I would mostly want a box that I could turn knobs and get a spectrum.

Well that's their old (1980's) version. It's got a decent low noise front end. They make a newer one,

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~$10k. Audio market. :^)

George H.

Reply to
George Herold

I have an HP 35665A that I love. I paid $350 for it about 8 or 9 years ago, and it works flawlessly.

They're very good for tuning analogue control loops, for instance, or for measuring the transfer function of some slow plant such as a TEC/thermistor or voice coil/LVDT.

It also has swept sine and Gaussian white noise sources and a bunch of other useful functions. Plus it runs good old Rocky Mountain Basic, so you can use it to control other instruments. (Shades of grad school.)

I got the All Options ROM from GLK Instruments for $50, which unlocks all the built-in goodies. They have to customize it for your unit's serial number.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Try

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Prof78

Reply to
Prof78

I'd be loathed to pay anyone $50 for 30 seconds of their time programming a 50c part.

I upgraded mine thanks to the instructions shared by George.

Hi all, This topic about the eeprom in the HP 35665A seems to be quiet for sometime. I thought that I would share my recent findings about the 35665A options. A few months ago, I purchased a like new 35665A with all documentation except the service manual. It had only one option ANA. Unfortunately, I have not been able to locate the service manual or options disks and there is no support from Agilent. After some detective work and from the post #21350 and others, I was able to active all of the options.

The bits to activate options 1D4, 1D3, 1D2, 1D1, 1D0, 1C2, and 1C1 are stored at hex address locations 23h, 24h, 25h, 26h, 27h, 28h, 29h, respectively. To active all options or any option, fill the address location(s) with 76h. Here is the eeprom hex dump from my instrument:

0000: 00 13 20 5D 04 05 06 00 08 09 48 50 33 35 36 36 .. ]......HP3566 0010: 35 41 20 20 33 34 34 35 41 30 32 36 36 36 1E 1F 5A 3445A02666.. 0020: 20 21 00 76 76 76 76 76 76 19 2A 2B 2C 2D 2E 2F !.vvvvvv.*+,-./ 0030: 48 45 57 4C 45 54 54 2D 50 41 43 4B 41 52 44 20 HEWLETT-PACKARD 0040: 33 35 36 36 35 41 20 44 59 4E 41 4D 49 43 20 20 35665A DYNAMIC 0050: 53 49 47 4E 41 4C 20 41 4E 41 4C 59 5A 45 52 20 SIGNAL ANALYZER 0060: 60 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F `abcdefghijklmno 0070: 70 71 72 73 74 75 41 77 78 79 7A 7B 7C 4D 48 99 pqrstuAwxyz{|MH.

I will also upload the binary file. With a hex editor, you can edit the binary file and put in your instrument's serial number in place of 3445A02666 although this is not necessary. Remember that options 1C2 and ANA are mutually exclusive.

Also, the 85C72 eeproms are obsolete (and expensive if you order from IC dealer who specializes in obselete ICs). Use Microchip's 24C01 eeprom - they are $0.29 from Mouser. They are equivalent and work fine in the 35665A. Also, you might want to replace the lithium battery (B200 on the main board) while you have the instrument open.

Regards, George

Reply to
JM

You're just cheap, is all. $50 to a public benefactor like that is money very sell spent.

Thanks for sharing the instructions.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Very few, in fact. There's a whole range of types, your review was very limited.

Mostly it's dynamic range, and image rejection.

Zero-IF converters get a lot of LO2 feedthrough.

Low-IF still has a number of image problems unless you have very good front-end filters (and few SDRs do).

8-bit converters (like in RTL-SDR) might be adequate to receive digital TV, but nearly useless as an SA.

Really wide-band front-ends have massive noise problems that makes wide dynamic range a near impossibility.

In short, the better hobbyist-grade SDRs are good to perhaps 40dB, whereas pro equipment goes beyond 70dB.

The hobbyist community seems blissfully unaware of many of these problems, because they don't own and perhaps have never even used pro-quality instruments, and don't understand why they are so complicated, with cavity resonator filters and banks of switchable or electronically-tuned filters.

Clifford Heath.

Reply to
Clifford Heath

Listen to

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with 16 bit sampling of 0 to 29 MHz.

Use "View Spectrum" to get the traditional spectrum display instead of waterfall display. Left click on the frequency scale to change frequency.

Unfortunately the receiver is on a campus with high background noise and of course there is a lot of band noise on HF and especially on MF/LF, so the true potential of the ADC is hard to determine. The strongest signal seems to be a nearby pager at 27 MHz at about 0 dBm.

At upper HF the background noise is about -100 dBm in 3 kHz bandwidth. To me, this works surprisingly well for its intended purpose, but of course, this is not a calibrated test instrument.

The problem is that in past decades quite complex structures were needed for spectrum analyzers and general coverage receivers (such as multiple PLLs). Today, when good components are available, such as NCOs, DACs and ADCs the same performance can be achieved with less complexity and hence less cost.

Reply to
upsidedown

Not the same performance. Adequate for many tasks, but no way the same. The best wide-band SDRs have big banks of RF filters, and they're still not as good as a proper communications receiver. Good enough to make pretty waterfall displays, but no way the same.

Reply to
Clifford Heath

For low frequency high res I have a National Instrument DAQ 24 bit aquisiti on unit:

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And a cheap SW I got for 100 USD that does all sorts of tricks

I can dig up signals buried 100dB below a carrier

I actually have one unit for sale for highest bidder ?

Cheers

Klaus

Reply to
Klaus Kragelund

The weakest point in both SA as well as general coverage receivers and scanners is the mixer.The situation can be somewhat helped by using special mixer structures and/or using a lot of power (not possible in a portable device).

In addition, if you are driving a mixer with a square wave, there will be mixing products between the odd harmonics of the LO and input spectrum at the IF distance of the odd harmonics of the LO. For this reason, octave filters are used to keep frequencies close to the 3rd harmonics (and higher) out of the mixer.

For a SA with 1 kHz to 3 GHz range, you will need more than 20 octave filters to cover the range. The good thing is that you can attenuate each octave separately and fix the corresponding attenuation in the display. In fact suboctave front end filters might be helpful to filter out second order mixing products of two strong input signals at the lower end of the octave, which would fall on the upper end of the octave.

However if the upper frequency requirement is more modest (say HF/VHF), you could get away with the problematic mixer and run the signal directly into the ADC.

It should be noted that a 16 bit ADC the theoretical weak signal resolution is 97 dB, there are always going to be band and front end noise, which work as dither, making it possible to resolve signals well below 100 dB FSD.

While the problematic mixing has been used for decades, the performance of ADCs will constantly increase, moving the practical dividing line to higher and higher frequencies.

Reply to
upsidedown

Mixers are non-linear devices. Even a perfect mixer will still have the same image problems that plague real receivers.

Zero-IF receivers are also afflicted by 2LO, not just 3LO, because the frequencies "fold around" zero.

There's quite a good review of available devices in the slides here (though these still pass over some of the problems):

Clifford Heath.

Reply to
Clifford Heath

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