[Justin Crooks]

Ivan,
The BB60C spur reject option will generally remove input-related spurious (signals generated from image frequencies, harmonic mixing products, etc.). What it cannot remove is residual signals from internal clock multiples. It also cannot remove intermodulation products from the input itself (e.g. strong 101 MHz and 103 MHz produce low-level 99 and 105 MHz IM3).
If you see the signal with no input, it is probably a residual clock multiple (typically 10 MHz intervals, sometimes stronger 80 MHz multiples), which unfortunately cannot be removed. These should be below -106 dBm at maximum sensitivity, and are typically around -120 dBm except for a couple of spots.
If you suspect intermodulation products, increase the reference level. This will decrease artifacts from intermodulation.

• This reply was modified 9 years, 4 months ago by Justin Crooks.

Thank you for bringing this to our attention. It looks like there is a software bug we will need to address. I’ll let you know what we find.

You could do this manually, by setting the TG frequency and amplitude directly, rather than SNA mode, or even write a small application to automate this.

I would love to add a suite of tools to the SNA for doublers, triplers, mixers, etc. but we do not currently have these features available in Spike.

The VSG’s odd-order harmonics at 100 MHz distort the time domain waveform, making it look like a high-passed square wave, but if you use an external 150-200 MHz low pass filter to remove the harmonics, you can easily clean it up and get a waveform that looks good in the time domain. In the manual we talk about applications that are sensitive to harmonics will require a low pass filter. This is one of those applications.

The TG124A also produces significant harmonics above 16 MHz, but the SA124B removes the harmonics, giving you a good measurement.

A good filter for applications from 100 MHz to 200 MHz requiring low harmonics is http://www.minicircuits.com/pdfs/BLP-200+.pdf

Any application where the output of the VSG goes to a receiver or analyzer will generally not be sensitive to harmonics. In fact, for clocking applications, the harmonics may actually be desirable in many cases to improve slew rate.

I’m glad you find it useful. I wish the 5 MHz RBW had better performance on the SA44B (DANL / spurious / amplitude accuracy), but I suppose for a quick, low res sweep it gets the job done.
The 200 MHz cutoff is necessary because of extremely poor spurious performance at lower frequencies.

We don’t have any VNA hardware or software support. In theory, you could measure your filter’s pass band phase response using a VSG25A, BB60C, and some fancy software that does not currently exist. The multi-tone feature on the VSG, with parabolic phase tones (for low peak-to-average power ratio), combined with the BB60C streaming I/Q data, could be combined in a clever way to measure phase response, easily up to 25 MHz, or wider by patching several results together. It would take some work…

I emailed you the firmware upgrade to 3.03. If that doesn’t work, it may require repair.

Saira,
GMSK modulation is FSK modulation, modulation index 0.5, Gaussian filtered. Access it from the ASK/FSK controls.

1. Yes. Powers of 2. 16, 32, 64, 128, …
2. Yes.
3. Number of points varies a little based on window bandwidth. For the legacy API, it is closer to 3.5 * span / RBW
4. 440.4 to 440.8 MHz would probably be 2 slices (440.4 – 440.6, 440.6 – 440.8). If each slice is 56 ms, your sweep time would be 112 ms.
5. The API expects FFT size. The user interface expects RBW. Use the relationship from question 2.
6. IF bandwidth is set using decimation (IF BW = 240 kHz / decimation)
7. I would expect this to work. Configure, sweep, configure, use measuring receiver (actually more of a modulation analyzer).
Let me know if you have more questions.

Also, below 28 MHz, the output is DC coupled (as it goes down to 10 Hz). This small DC offset has a 50 ohm impedance, so it adds a little to total envelope power, but will not damage the SA44B input.

Ivan,
The TG44A produces 10 Hz to 4.4 GHz, -30 dBm to -10 dBm. This is why 0 dBm and -40 dBm do not read accurately–they are beyond the amplitude range of the device.

The TG44A, as a tracking generator (not a signal generator), has a very simple job: generate a signal that tracks the SA44B’s input frequency, at a specific frequency and a repeatable amplitude. Since the SA44B can filter out harmonics, we do not filter them on the TG44A. This reduces cost, size, weight, and power, without really affecting performance as a tracking generator.

Below 28 MHz, the frequency is generated with a DDS, thus the sinusoidal output. Above 28 MHz, it is produced by a clock or divided LO, and will be closer to square than sinusoid. A external low pass filter can easily remove harmonics to produce a sinusoid if needed.

Yes, there is. It is aptly named the TG standalone API. Email support at signalhound dot com for a copy of this.

Ivan,
The actual value of the 20 dB pad is not important. It is measured and nulled as part of the store 20 dB procedure. We just need enough attenuation so we don’t overdrive the ADC when setting it to the high gain state. This is usually 12-16 dB minimum. A 30 dB pad would even work, but the readings would be noisier.

The BB60C residual responses are specified to be below -106 dBm, are typically below -110 dBm (at -50 dBm ref level) and most of them go away when you turn “spur reject” on. Unfortunately, it is difficult to do much better at our price point, and impossible to eliminate spurious / residual responses entirely, at any price.

Thank you for the suggestion. Our solution for frequency hopping spectrum analysis is the BB60C. It has hardware image rejection and very fast sweeps.
There are a number of solutions for frequency hoppers using the SA44B, but they are generally case-specific and do not work in the general case. If you can keep the hopping to a 20 MHz band, the SA44B can be used with image rejection off (disregard the image responses ~21.4 MHz away), or if the transmissions are long at each hop (>sweep time) you can keep image rejection on and analyze 40 MHz worth of spectrum.
Beyond this, the BB60C is required.

BBowar,
The USB-SA44B has higher LO feedthrough than other models we sell, and the LO is offset by only 10.7 MHz. You can minimize it by setting a reference level around -30 dBm (which will turn on the RF preamplifier), or manually turning gain to 1, preamplifier on, attenuator to 10 or 15 dB. This should reduce the LO feedthrough to around -60 dBm (typical).
Choosing a product like the BB60C will keep LO feedthrough below -80 dBm, and will keep the LO > 1.2 GHz away from the RF.
You can also add a circulator, ore even an external attenuator and/or preamp, to reduce LO feedthrough if needed.

The SA44B would work for measuring steady-state noise, such as from a clock, but intermittent noise would not measure accurately. The BB60C would be the recommended product for measuring intermittent / pulsed noise.

Ryan,
Unfortunately, spurs like this are normal for the USB-SA44B. The frequency is usually a little higher, but it depends on your cable lengths and computer. It looked like your spurs peaked around -100 dBm with a -30 dBm reference level for a 100 MHz span.

When you use a span of 200 kHz or less, most of these spurs will be drastically reduced or eliminated. This is because the SA44B mixes differently for narrow spans.

The best workaround is to show multiple captures. You could do several 200 kHz steps with low RBW. If you wanted you could export them as CSV, paste them together, and then plot using spreadsheet software or your favorite plotting tool.

I realize the RBW/span limitation at low frequencies seems odd, but the combination of hardware limitations at those frequencies, and the sweep performance we wanted for Spike, made it necessary.

With a shared external frequency reference, in streaming mode only, there should be zero phase drift. However, every time you change center frequency, there will be a new phase offset.

If there is phase drift from floating point frequency correction rounding errors at some frequencies, I would think it would be very small and identical across devices and therefore cancel, but I have not tested this (maybe 1 degree per minute?).

At even multiples of 20 MHz (e.g. 200.00 MHz), this correction should be zero, so you should get a true zero phase drift. If you test this, let me know. If there is phase drift, there should be a way to easily get rid of it, possibly by using streaming IF instead of I/Q…

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