Some information on the LNA 10 low-noise oscilloscope preamplifier

[dnessett]

I recently purchased two LNA 10 low-noise oscilloscope preamplifiers to use in an application I am working on - measuring phase noise in some hobbyiest oscillators. In preparation for using them in this application, I did some simple testing, the results of which I thought I would share.

This is not a full or even a partial review of the LNA 10. Rather, it comprises some notes that I thought others might find useful, if they decide to purchase the device.

For the cost, $270, the LNA 10 is an impressive piece of equipment. It provides 10X, 100X and 1000X voltage gain with an implied bandwidth of 1Hz - 1MHz. I say this is the implied bandwidth because, rather surprisingly, the bandwidth is not actually specified in the documentation. However, it provides a selectable analog low-pass filter on the input that may be set from 1 Hz to 1 MHz, so the bandwidth should be within this range. Noise is less than 4 nV/sqrt(Hz) for offset frequencies > 100 Hz. For offset frequencies less than 100 Hz, its noise characteristics are:

Freq	Noise(dBm)
10-100 Hz	4.1 nV/sqrt(Hz)
1-10 Hz	6 nV/sqrt(Hz)
10-2-1 Hz	20 nV RMS
10-4-10-2 Hz	50 nV RMS

Note that the last two noise values are expressed in terms of RMS noise, rather than spectral density noise.

Other features of the device are:

• Selectable DC/AC coupling on input
• One-sided or differential input options
• Input impedance : 500 Kohm
• Output impedance : 470 ohm
• Impedance between differential inputs - 1 Mohm
• CMRR on differential input : > 90 dB
• Gain accuracy : +/- 1%
• Offset adjustment, equivalent to +/- 1mV at input

In order to determine the suitability of the LNA 10 for my application, I ran some tests on its performance. I used a Rigol DG-1022 signal generator to produce a fixed frequency sine wave; a PicoScope 4262 in spectrum mode to analyze the results; and the LNA 10 to provide amplification. For the case of no gain, I connected the output of the DG-1022 directly to the PicoScope.

The goal of the first test was to determine the gain characteristics of the device at various frequencies and gain settings. The parameters of this test were:

Test Parameters

For all configurations - PicoScope: 16,384 bins; 30 segment averaging; Blackman-Harris Window; no-termination at PicoScope input; SG amplitude: 1 mV_P-P

1 KHz
LNA: 2 KHz LPF
PicoScope: 1 - 5 KHz span; 10 KS/s; 3.277s time gate

10 KHz
LNA: 20 KHz LPF
PicoScope: 1 - 20 KHz span; 40 KS/s; 819ms time gate

100 KHz
LNA: 200 KHz LPF
PicoScope: 1 - 200 KHz span; 400 KS/s; 81.92ms time gate

500 KHz
LNA: 1 MHz LPF
PicoScope: 1 - 1 MHz span; 2 MS/s; 16.38ms time gate

Test Results

Freq	No Gain (dBV)	10X gain (dBV)	100X gain (dBV)	1000X gain (dBV)
1 KHz	-57.41	-38.41	-18.3	1.509
10 KHz	-57.36	-38.36	-18.42	1.508
100 KHz	-57.42	-38.85	-19.02	0.978
500 KHz	-57.43	-38.92	-19.43	0.782

Freq	No Gain (dBm)	10X gain (dBm)	100X gain (dBm)	1000X gain (dBm)
10 KHz	-55.13	-35.92	-16.04	3.97

I setup the PicoScope to use log amplitude averaging for the first set of results (using units of dBV) and also ran a test on 10 KHz using log power averaging (using units of dBm). In both cases the change in values were roughly +20 dB for each increasing gain setting. I ran the dBm test, since in my application the measurements of interest are log power values.

Note that the LNA 10 yields fairly consistent gain characteristics for 10X gain. For 100X and 1000X gain, there is some roll-off in the gain at higher frequencies (100 KHz and 500 KHz).

One thing I ran across when playing around with the LNA 10 was an undesirable property that the noise inherent in the DG-1022 signal was not amplified the same amount as the coherent signal. Figures 1-4 are PicoScope plots for a 50 KHz signal amplified as follows: Figure 1 - no gain; Figure 2 - 10X gain; Figure 3 - 100X gain; and Figure 4 = 1000X gain.


Figure 1 - Signal with no amplification

Figure 2 - Signal with 10X amplification

Figure 3 - Signal with 100X amplification

Figure 4 - Signal with 1000X amplification

Notice that the noise floor in the 10X amplification case is about 50 dB greater than in the no gain case. For the 100X amplifcation case, the noise floor is about 20 dB greater than the 10X amplification case. For the 1000X amplification case, the noise floor is about 10 dB greater than in the 100X amplification case. This is a serious problem for my application unless a solution was available.

Fortunately a solution exists. I contacted AlphaLab and asked them about this problem. They replied as follows (I am quoting them with their permission):

To avoid high current surges (if there's a voltage difference between input and output grounds), the input grounds have a series 10Ω resistor. When using only the I+ (or I- alone) input, any ground noise is amplified. This can be substantial noise, because the Picoscope ground and signal gen ground form a loop. You can even see this if you set the knob to I+, and short the I+ input, then touch it with your finger, or touch the signal gen ground to the I+ ground. This excess noise is completely eliminated if you short the I-  input, connect the signal to I+ , and set the knob to I+ - I-.

Best regards,
Bill Lee
AlphaLab, Inc.

I created a simple shorting device by cutting the BNC connector off of an RG-58 cable and soldering the signal wire to the grounding mesh. I then connected this device to the I- input of the LNA 10, connected the signal to the I+ input and used the I+ - I- input option. The result was much better. Figures 5-7 show the spectra produced by the PicoScope for the cases: Figure 5 - 10X gain; Figure 6 - 100X gain; and Figure 7 = 1000X gain.


Figure 5 - Signal with 10X amplification

Figure 6 - Signal with 100X amplification

Figure 7- Signal with 1000X amplification

In each case, the noise floor is amplified by approximately an additional 20 dB for each increased gain setting.

The next step for me is integrating the LNA 10 into the procedure for using a phase detector (in my case an HP11729C) to analyze phase noise in some hobbyist oscillators. (For those interested, see this topic)

[_Wim_]

Thanks for the results. Very interesting! Did you have a look inside the device? Teardown pictures would be nice so see...

[dnessett]

Thanks for the results. Very interesting! Did you have a look inside the device? Teardown pictures would be nice so see...

Since the device is so cheap, I will leave it to others with more expertise in LNAs to do a teardown and detailed review.

[thm_w]

$270 may be cheap to some but not others

It seems to be very similar to the EE701, but a much nicer case:
https://www.ee-quipment.com/products/differential-preamplifier?variant=35410631368
https://cdn.shopify.com/s/files/1/0362/5057/files/ee701_product_overview.pdf?17224356681419949916

Your other thread is interesting as well: https://www.eevblog.com/forum/metrology/need-lna-for-low-frequency-low-level-preamp/ someone linked a design using AD4898

[dnessett]

$270 may be cheap to some but not others

It seems to be very similar to the EE701, but a much nicer case:
https://www.ee-quipment.com/products/differential-preamplifier?variant=35410631368
https://cdn.shopify.com/s/files/1/0362/5057/files/ee701_product_overview.pdf?17224356681419949916

Your other thread is interesting as well: https://www.eevblog.com/forum/metrology/need-lna-for-low-frequency-low-level-preamp/ someone linked a design using AD4898

Thanks for the links to the EE701. I was not aware of that product.

In regards to a teardown/review of the LNA 10, I simply do not have the background and expertise to do a proper job on that. I will leave it to those who do.