Bergoz Log-Ratio BPM Tests on ALICE and VELA
A. Kalinin 150302
1. We have available two cards of a commercial Log-Ratio BPM from Bergoz Ltd. Longpending tests of this type BPM on our beams has recently been done. The results are
presented below.
2. This BPM designed fifteen years ago (by me) and still available on market belongs to a
family of Log-Ratio BPMs based on log amplifiers from Analog Devices. These simple and
inexpensive BPMs are intended for undemanding beam position monitoring in transfer lines,
linacs, dumps, etc. They have a moderate accuracy and resolution. From other side, they don’t
need a switchable attenuator at the input, and have convenient internal
triggering/synchronisation.
3. The Log-Ratio BPM has usually a narrowband filter on the input (typically tuned to 500
MHz). The filter ringing response is detected by a log amplifier. Difference of opposite
pickup electrode signals 𝑉1(2) and 𝑉3(4) at the log amplifier outputs is proportional to the
beam offset 𝑢(𝑤) along the 𝑢-(𝑤-) axis of the electrode pair:
𝐾 ∙ (𝑙𝑜𝑔
4.
5.
6.
7.
𝑉1(2)
𝑉3(4)
𝑉1(2)
1 ± 𝑢(𝑤)
− 𝑙𝑜𝑔
) = 𝐾 ∙ 𝑙𝑜𝑔
= 𝐾 ∙ 𝑙𝑜𝑔
≅ 𝐾 ∙ 2 ∙ 𝑢(𝑤)|
𝑉0
𝑉0
𝑉3(4)
1 ∓ 𝑢(𝑤)
𝑢(𝑤)≪1
Transformation of the own pickup axes u, w to the x-, y-axes in the Bergoz card is done using
OPA-based circuit.
Under our request, the Bergoz card filter was factory-tuned to 406 MHz = 81.25 MHz x 5.
The card can work with ALICE trains of any bunch rate 81.25 MHz / n, 𝑛 = 1, 2, 3, … The
card is able to work with single bunch albeit in this case re-adjustment of the scale coefficient
and zero offset is needed.
For the given pickup, the card can be adjusted to have a desirable scale coefficient (say, 10
mm per V).
A characteristic feature of a Log-Ratio BPM is that without beam it exhibits at the output a
large noise. The beam signal causes compression of the noise. The Log-Ratio BPM resolution
is typically not lower than 100 μm.
One card was adjusted for ALICE (trains), another one for VELA (single bunch). A card is
shown in Fig. 1.
Figure 1. Bergoz Log-Ratio BPM card (Eurocard 3U 4HP, the cover is removed). The four input
connectors are top right, each one is connected through a filter (double helix) to a log amplifier and a
buffer OPA.
A. ALICE
1. A pickup ARC1-pickup3 was used. It has two pairs of buttons up and down. Not bothering
about pickup geometry, I read the signals of the pickup own axes. The card was placed at
EMMA Rack Room, and connected to the pickup through four ECOFLEX-10 cables (length
about 50 m). The signals were observed at LeCroy HDO6104 oscilloscope.
2. No interference noise was observed at the outputs.
𝑉
3. The card produced three signals: 𝑢, 𝑤, and ∑ 𝑙𝑜𝑔 𝑉𝑖 . The latter was used to trigger the
0
oscilloscope. Bunch charge was around 60 pC. The signals are shown in Fig. 2.
𝑉
Figure 2. Train 100 μs (shift 150212, stable lasing). The signal u olive, w bourgogne, ∑ 𝑙𝑜𝑔 𝑉𝑖 salad.
0
Before and after the train pulses a large noise mentioned above is seen. For < 0, 𝑤 > 0, and |𝑢| ≈
|𝑤|, the train has 𝑥 > 0, 𝑦 ≈ 0 which is typical for ARC1. A rough estimate gives 𝑥 ≤ 6 mm.
4. In Fig. 3, a 1 μs train is shown, with vertical zoom. The ripple is seen produced by separate
bunches (bunch rate is 81.25MHz / 5 ~ 16MHz). Pay attention at a position change after
seventh bunch. Using a simple LPF, the ripple can be suppressed.
Note here the BPM can’t resolve separate bunches, it ‘remembers’ previous bunches with lag
about three periods which results in residual ripple.
Figure 3.
𝑉
5. In Fig. 4, a blue trace is the bunch intensity in linear scale. The ∑ 𝑙𝑜𝑔 𝑉𝑖 is transformed to
0
𝑉
∑ 𝑖
𝑉0
𝑥
using oscilloscope-built-in 𝑒 function.
Figure 4.
6. Fig. 5. The same as above, with zoom to see position and intensity variation.
Figure 5.
7. See Fig. 6. Again a train 100 μs. In yellow, the spectrum is shown (0 to 20 MHz, DC offset is
subtracted). A peak on the right is produced by repetition of bunches with 16 MHz. A zoom
of the (0 to 1 MHz) region is shown in Fig.7. It was observed that the peaks 100 kHz and 300
kHz notorious in the past can be recognised: here they are repeatable but noise-like. It appears
that the BPM is not sensitive enough to resolve the spectrum of the position variation which
at the present ALICE looks quite low.
Figure 6.
Figure 7. The FFT (yellow, 0 to 1 MHz) is calculated for the part between the markers. The FFT
resolution is about 20 kHz.
8. It looks that one or two of such BPMs would be useful on ALICE, to monitor the trains on
their length in a quest for ultimate FEL stability. The train signals can be easily digitised.
With some ordinary ADC clocked by, say, 81.25 MHz the FFT resolution would be 10 kHz
for 100 μs train.
B. VELA
1. Bergoz Log-Ratio BPM is not suitable for VELA/CLARA. The test on VELA was done with
VELA as simply a source of single bunches.
2. In single bunch mode, the difference of log amplifier outputs goes through a chain T&H and
S&H triggered by a built-in beam-based trigger circuit. The S&H keeps the 𝑢-(𝑤-) output
𝑉
constant up to the next bunch. The ∑ 𝑙𝑜𝑔 𝑉𝑖 signal is a direct one and has a shape of the filter
0
ringing response envelope.
3. The ILC pickup was used adjacent (through a bellow) to a pickup of BPM5. It has two pairs
of 𝑥-, 𝑦-strips. The card was placed at VELA Rack Room, and connected to the pickup
through four ECOFLEX-15 cables (length about 50 m). The signals were observed at Agilent
MSO-X 3054A oscilloscope.
4. No interference noise was observed at the outputs.
𝑉
5. The card signals 𝑥, 𝑦, and ∑ 𝑙𝑜𝑔 𝑉𝑖 are shown in Fig.1 in blue, pink, and olive respectively.
0
Green is the trigger pulse. The latter was used to trigger the oscilloscope. Bunch charge was
around 50 pC.
Figure 1. The signals 𝑥, 𝑦 (blue, pink): the S&H internal transients in the middle occur when the S&H
samples the signal of a new bunch. Difference of previous and new values shows the position jitter.
6. Fig. 2 shows the data for BPM5 (not the same shot as Fig. 1). The bunch offset is 𝑥 =
+5.6 mm, 𝑦 = −3.6 mm. One can see a big horizontal jitter, around ±0.7 mm, that took
place at this shift. The vertical jitter was within ±0.3 mm.
7. Combining the measurements above with the measurements of other bunch offset 𝑥 =
−0.9 mm, 𝑦 = +0.2 mm (shown in Fig. 3), one can find the scale coefficient and zero offset
of the BPM under test. For the vertical plane (where the jitter was lower) 𝑀𝑦 =12.7 mm/V
and Δ𝑦 = +2.3 mm. The offset is big because the pickup offset is outrageous which is seen
by naked eye.
Figure 2.
Figure 3.
C. References
[1] Log-Ratio Beam Position Monitor. User’s Manual. Rev. 2.2.0. Bergoz Ltd.
[2] A. Kalinin, Log-Ratio Beam Position Monitor, BIW’02, AIP, 2002, p. 384.