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Technical

March 2018

77

-120dBm at 1.5MHz. 100kHz noise improves by

10dB if the output capacitance is raised to 100µF,

but LF noise needs even more to reduce it.

2200µF

was needed to reduce it by a further 10dB in the

10kHz region.

Input noise was measured as -100dBm at

100kHz and could be reduced by increasing the

input capacitor to 100µF, as expected. BUT, this

noise on the

input

terminal was made worse by

any increase in the

output

decoupling capacitor.

So both capacitors have to be varied together.

Figure 2

shows the result for 100µF on the output

and 10µF on the input. A further 2200µF had to

be placed on the input to reduce noise to close to

the receiver’s measurement limit.

The device oscillated in the LF region with

anything less than 1µF on the output and 470nF

on the input.

LM317

The datasheet specification says

“1µF on the input if spaced far

from the PSU filter; 0.1µF on

the output is optional to improve

transient response with typically

1µF to 1000µF as needed”.

0.68µF was used here.

Noise is worst at low

frequencies, with a definite peak

at 84kHz as shown in

Figure 3

.

But the noise does roll off faster

with this device, dropping to

-110dBm at 300kHz, -120dBm

at 500kHz and down to the

measurement limit at 1.2MHz.

Adding extra output C shifts the

LF peak lower: a value of 100µF

drops the noise at 100kHz to

-110dBm, while 2200µF takes

it nearly to the measurement

limit.

Although not normally

advised, some applications

bypass the adjustment pin to

ground to reduce noise. Testing

this with an extra 0.68µF in

that position flattened the low

frequency response of Figure 3

at a level of -100dBm over

a wide band. Proper noise

reduction and decoupling in

the LM317 can become quite

complex, but one thing that

must

never

be done is just to

connect a capacitor between

output and the adjustment pin.

A capacitor here will

definitely

make it go unstable

[3].

Apart

from this scenario, the LM317

could otherwise never be made

to go unstable and oscillate.

Conclusions

All three voltage regulators

tested

generate

quite

appreciable levels of noise at

LF to MF frequencies when using the minimum

decoupling specified in the manufacturer’s

datasheets. It is easy to see why they give their

greatest problems with sensitive PLL circuitry and

are also regarded badly in the Hi-Fi audio world.

Of the two fixed voltage ones, the low dropout

LM1117 fared quite a bit worse at LF but did roll

off slightly faster as frequency rose into the HF

region. The adjustable LM317 with minimum

decoupling fared badly at lower frequencies, but

rolled off faster above 300kHz. It also cleaned

up the best of the lot with extra C added on the

output – exactly as the datasheet suggested would

happen.

To properly clean up the resulting supplies

requires large values of additional capacitors on

the output. Some extra series resistance and/or a

choke and a large C to form a low pass filter are

going to be needed in critical applications.

Noise appearing on the regulator input pin

is seen in some devices but should not cause

a problem unless the input supply line goes

elsewhere to sensitive circuitry, where conducted

noise can leak through. This was the issue seen

during the loop amplifier tests where the DC input

to the LDO was provided ‘up the coax’. The noise

got into the RF feed to the receiver. A bigger input C

and optionally a series inductor improved matters.

Paul, M1CNK, who was one of the

experimenters who found the noise on the loop

amplifier, commented that “…the main issue,

apparently, is with noise coming from the voltage

reference. Therefore the better [low noise]

regulators have a separate pin for decoupling the

voltage reference. That way the capacitance used

for decoupling is not inside the control loop. One

problem with just adding larger C to the input and

output is that these become part of the control

loop. Hence by adding too much, you slow down

the response of the regulator to noise spikes – so

they just pass through”.

References

[1] All noise levels were measured in dBm; the power

delivered into the 50Ω input of the SDR-IQ in a

measurement (FFT) bandwidth of 24Hz. The SDR-IQ

has a minimum sensitivity of around -128dBm at this

setting but a lot of local computer interference is picked

up at frequencies below 100kHz so this sensitivity could

only be achieved above 500kHz. Fortunately (or not)

the voltage regulators under test exceeded even this

pickup level when using the minimum recommended

decoupling. The QRM was also spiky, so ‘real’ noise could

be discerned between the spikes.

To convert a measured dBm reading to a voltage, first

normalise from the 24Hz measurement bandwidth by

subtracting 10.LOG(24) = 14dB from the dBm value.

Then calculate the voltage needed across a 50Ω resistor

to give this power. -100dBm measured = -114dBm/Hz

or 4x10

-15

watts per Hz. In 50Ω this corresponds to

450nV per root-Hz.

[2] The tantalum capacitor was hand-held in my fingers

while it was placed across the regulator terminal pins for

testing. I got it the wrong way round. It is at times like

that you really appreciate what happens when a tantalum

bead capacitor is connected with the wrong polarity!

[3] It is easy to fall into this trap unintentionally, as a

capacitor directly across these two pins is often seen with

7805 type devices. So it can be tempting to just place

one when designing and laying out PCBs. The story may

be apocryphal, but a certain spacecraft manufacturer on

the south coast apparently fell into this trap many years

ago. It was only when a PSU badly failed EMC testing

(fortunately while still at the breadboard stage) that the

mistake was found.

Andy Talbot, G4JNT

andy.g4jnt@gmail.com

FIGURE 2:

Output noise from the LM1117 LDO regulator when

decoupled with 100µF on the output and 10µF on the input. Input

and output decoupling capacitors have to be increased together to

control noise from this device.

FIGURE 3:

LM317 adjustable regulator with the minimum

specified values of 1µF on the input and 680nF on the output.

The noise peak at 84kHz reduces in frequency as the output

decoupling C is increased, but values up to 1000µF or more are

needed to get it right down to insignificant levels.